I picked up a great web.config workaround from Kent Sharkey's talk at the Victoria, BC code camp on Saturday.
If you're editing the web.config file in Visual Web Developer Express and not getting IntelliSense support, Try this:
Get rid of the xmlns namespace for the <configuration> element.
Instead of this:
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
use this:
<configuration>
Bam! IntelliSense kicks in and you can stop guessing at the syntax.