Several time the web designer failed to load controls correctly, claiming that it couldn't locate the containing assemblies. In looking underneath, I discovered that the build system wasn't copying the dependent control assembly DLLs to the target project's Bin directory-- the files instead went into the Debug or Release directory. Unfortunately, the designer didn't look there, but in the upper-level Bin directory.
Copying the files by hand corrected the problem. Even removing/restoring the inter-project dependencies didn't fix the problem.
This is only the most recent of several serious problems I've found with the build system in VS2005 ASP.NET projects (see previous posts). Sad to say, this product is not anywhere near the quality of VS2003's initial release. The build system is ponderous, inaccurate and prone to endless, needless rebuilds. And what is "Verifying Web Site" doing other than taking up my time? This step was not necessary in VS2003.