You could use the Panel solution proposed above by giving the UniqueId of the Login control submit button. For example, if the ID of the Login control is "Login1" then the panel would look like this:
<asp:Panel ID="Panel1" runat="server" Height="50px" Width="125px" DefaultButton="
Login1$LoginButton">
<asp:Login ID="Login1" runat="server">
</asp:Login>
</asp:Panel>
If this doesn't suit your needs, you could set the default button by means of JavaScript. Take a look at post
http://forums.asp.net/507628/ShowPost.aspx for sample code on how to do this.
Hope it helps,
- Federico