I recently installed Resharper 3.0.2 for VS2008. I found that many of my OnClick etc. handlers were marked as errors as the symbols could supposedly not be found. The project compiles fine though. IntelliSense also stopped working properly on many symbols.
After lots of searching I found the cause of the problem - since this issue remained even when I reverted back to VS2005.
http://intellij.net/forums/message.jspa?messageID=5202694#5202694
The essence of the problem is that VS2008 sometimes adds this to your web.config file:
<controls>
..
<add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0., Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
..
</controls>
To solve the problem change the tag prefix asp to something else like aspajax.