If you ever wanted to try (or even use) Mono 64 bits on Mac OS X, I have created a small project on GitHub called Mono-Sixty-Four. This is a simple scrip that creates a minimal 64 bits Mono runtime and merges it with an existing Universal 32 bits Mono runtime. At the end, you get a Universal 32/64 bits Mono runtime.
For the moment, only the Mono runtime supports 64 bits. Additional libraries (GTK, SQLite, etc) ARE not supported.