I've read a few reports which have mentioned a problem with .NET's String.IsNullOrEmpty static method where under certain circumstances it can cause a null reference exception at runtime. Bill at MSMVPS.COM has as article with some sample source code that can be used to test this problem.
I've done some experimentation and found that it only occurs when a program is compiler in release mode and run outside of the development environment on a PC with .NET 2.0 installed. Debug builds are not affected and neither are computers running Mono.
It looks like this may be a bug in the optimizations performed by Microsoft's .NET CLR or JIT on Windows. Microsoft have posted an article addressing this issue and have stated that it will be fixed in the Orcas CLR beta 1.
Needless to say, the .NET developer community are shocked that a flaw in the CLR with such serious consequences as this will not be fixed in the current version, but instead people will need to upgrade to a new and relatively untested technology preview.