[updated: cleaned up links]
Ran into a bug in the shadow copy cache code in the technical preview of service pack 1 for the .Net Framework v 1.1 last week. After installing the TP I was no longer able to get NDoc to run (it began throwing InvalidCastExceptions when loading projects at startup).
Since I release CHM documentation with each build of my user facing code this was a bit of a problem.
Turns out this is a known regression in the TP build which will be fixed in the final release.
If this has bitten you the good news is that uninstalling the TP will fix the the issue, and that the dotnetfx group knows about the regression and will fix it in the release of SP1.