I have heard from a couple of customers who have seen package load failure errors in Visual Studio 2005 and for whom the workarounds I normally recommend did not work. We finally managed to figure out that the package load failures were caused by having Dotfuscator Professional Edition 3.0 installed and opening a solution that contained a Dotfuscator 3.0 project and other Visual Studio packages. This bug report describes some of the symptoms.
Preemptive has released an updated version of Dotfuscator 3.0 on 1/9/2006 that should resolve this issue. You can view the release notes for this updated version on this site. If you are running into package load failures in Visual Studio 2005 and you have Dotfuscator 3.0 installed, I encourage you to uninstall your current version of Dotfuscator and install the updated version provided by Preemptive to try to resolve this issue.