I ran across a known bug in MSBuild in Whidbey beta 2 that caused me considerable headaches today.  It seems beta 2 will not copy project references to the bin directory of web projects when compiling with MSBuild.  This issue will be fixed in the final release, but I thought some Google juice might help someone else find the definitive answer with less frustration than I had.  Here's a relevant forum post on the subject.

In the meantime, I'll simply add a copy task to copy the relevant assemblies to the bin directory.