I have discovered several problems with the treeview adapter.

First, NodeCheckChanged event is refired for every node that is checked in every postback.  I created a fix that involves creating a postbackeventreference in the BuildItem method of the TreeViewAdapter.cs file:

                if (((item.ShowCheckBox != null) && (item.ShowCheckBox.Value == true)) ||
                    (treeView.ShowCheckBoxes == TreeNodeTypes.All) ||
                    ((treeView.ShowCheckBoxes == TreeNodeTypes.Leaf) && (!IsExpandable(item))) ||
                    ((treeView.ShowCheckBoxes == TreeNodeTypes.Parent) && (IsExpandable(item))) ||
                    ((treeView.ShowCheckBoxes == TreeNodeTypes.Root) && (item.Depth == 0)))
                {
                    writer.WriteBeginTag("input");
                    writer.WriteAttribute("type", "checkbox");
                    writer.WriteAttribute("id", treeView.ClientID + "n" + _checkboxIndex.ToString() + "CheckBox");
                    writer.WriteAttribute("name", treeView.UniqueID + "n" + _checkboxIndex.ToString() + "CheckBox");
                    if (item.Checked)
                    {
                        writer.WriteAttribute("checked", "checked");
                    }
                    // Add event for checkchanged
                    writer.WriteAttribute("onclick", Page.ClientScript.GetPostBackEventReference(treeView, "c" + (Page.Server.HtmlEncode(item.ValuePath)).Replace("/", "\\"), true));
                    writer.Write(HtmlTextWriter.SelfClosingTagEnd);
                    _checkboxIndex++;
                }

 To capture the event, I added the following code to the RaisePostBackEvent:
 

	if (eventArgument != null)
                {
                    if (eventArgument.StartsWith("s") || eventArgument.StartsWith("e"))
                    {
                        string selectedNodeValuePath = eventArgument.Substring(1).Replace("\\", "/");
                        TreeNode selectedNode = treeView.FindNode(selectedNodeValuePath);
                        if (selectedNode != null)
                        {
                            bool bSelectedNodeChanged = selectedNode != treeView.SelectedNode;
                            selectedNode.Selected = true; // does not raise the SelectedNodeChanged event so we have to do it manually (below).
                            ExpandToSelectedNode();
                            if (eventArgument.StartsWith("e"))
                            {
                                selectedNode.Expanded = true;
                            }

                            if (bSelectedNodeChanged)
                            {
                                Extender.RaiseAdaptedEvent("SelectedNodeChanged", new EventArgs());
                            }
                        }
                    }
                    else if (eventArgument.StartsWith("p"))
                    {
                        string parentNodeValuePath = eventArgument.Substring(1).Replace("\\", "/");
                        TreeNode parentNode = treeView.FindNode(parentNodeValuePath);
                        if ((parentNode != null) && ((parentNode.ChildNodes == null) || (parentNode.ChildNodes.Count == 0)))
                        {
                            ExpandToNode(parentNode);
                            parentNode.Expanded = true; // Raises the TreeNodePopulate event
                        }
                    }
                    else if (eventArgument.StartsWith("c"))
                    {
                        string parentNodeValuePath = eventArgument.Substring(1).Replace("\\", "/");
                        TreeNode parentNode = treeView.FindNode(parentNodeValuePath);
                        if (parentNode != null)
                        {
                            Extender.RaiseAdaptedEvent("TreeNodeCheckChanged", new TreeNodeEventArgs(parentNode));
                        }
                    }

 I then commented out the the RaiseAdaptedEvent line in the UpdateCheckmarks method:
 
                        if (item.Checked != bIsNowChecked)
                        {
                            item.Checked = bIsNowChecked;
                            // Adding postback support for checkmarks
                            //Extender.RaiseAdaptedEvent("TreeNodeCheckChanged", new TreeNodeEventArgs(item));
                        }

 That line was the cause of the TreeNodeCheckChanged event firing for every checked node.

The second problem I found is the TreeViewAdapter doesn't raise events properly if the treeview is in a webcontrol.  This is due to how the RaiseAdapterEvent was created.. 
 

        public void RaiseAdaptedEvent(string eventName, EventArgs e)
        {
            string attr = "OnAdapted" + eventName;
            if ((AdaptedControl != null) &&
                (AdaptedControl.Attributes[attr] != null) &&
                (AdaptedControl.Attributes[attr].Length > 0))
            {
                string delegateName = AdaptedControl.Attributes[attr];
                MethodInfo method = AdaptedControl.Page.GetType().GetMethod(delegateName);
                if (method != null)
                {
                    object[] args = new object[2];
                    args[0] = AdaptedControl;
                    args[1] = e;
                    method.Invoke(AdaptedControl.Page, args);
                }
            }
        }

        public void RaiseAdaptedEvent(string eventName, EventArgs e)
        {
            string attr = "OnAdapted" + eventName;
            if ((AdaptedControl != null) &&
                (AdaptedControl.Attributes[attr] != null) &&
                (AdaptedControl.Attributes[attr].Length > 0))
            {
                string delegateName = AdaptedControl.Attributes[attr];
                MethodInfo method = AdaptedControl.Parent.GetType().GetMethod(delegateName);
                if (method != null)
                {
                    object[] args = new object[2];
                    args[0] = AdaptedControl;
                    args[1] = e;
                    method.Invoke(AdaptedControl.Parent, args);
                }
            }
        }

 I did some basic testing and it looks like referencing the parent works a lot better than referencing the page.

The last problem I have figured out yet..  but it is easy to reproduce.  I can't get nodes to expand in some of the demos.  For example go here:
http://www.asp.net/CSSAdapters/TreeView.aspx
Change the theme to 'Enhanced', then try to expand a node.  The nodes won't expand.