NeinMath is playing around with arbitrary precision integers, written in pure managed code, not using any unsafe stuff, and a bit faster than the build-in .NET type for integers with a few thousand bits.
I like your project and would like to add two improvements:
.NET 4.0 compat (which is one line change)
Conversion to and from byte[] (in order to do RSA computation from external sources)
I'm already working on these so let me know if you accept pull requests.