If you find that the very useful intellisense for web.config in Visual Studio 2005 has suddenly broken, check out the namespace declaration for the configuration section. Some tools (such as the Web Configuration Tool) add in the following everytime they edit the file, and this breaks the intellisense:
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
To correct it simply remove the namespace declaration thus:
<configuration >
HTH
Ian