One thing I've done to get around this problem is I made the following slight change in my web.config file:
<controls>
<add tagPrefix="aspAjax" namespace="Microsoft.Web.UI" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
<add tagPrefix="aspAjax" namespace="Microsoft.Web.UI.Controls" assembly="Microsoft.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</controls>
then all of my ajax controls have a prefix of aspAjax and not asp. This caused all of my red lines to go away. For some reason Visual Studio does not seem to like you using the asp prefix for other assemblies.