today I have updated ReSharper from 3.0 to 5.1 and Visual Studio crashed every time,
when I open my project.

In my project I have a reference to SvnSharp (http://sharpsvn.open.collab.net/). With the
pdb from the assembly sharpsvn.dll (sharpsvn.pdb) Visual Studio 2005 crashes every time, when
ReSharper 5.1 is active, with ReSharper 3.0 I have not a problem.

You must only rename the pdb file and then Visual Studio crashes no more.