Most likely cause is linking against a different version of Ruby than MacVim links against. MacVim links against the system Ruby by default (unless you built it yourself and took explicit steps to make it link against some other Ruby), so you have to build Command-T using the system Ruby. No RVM or any other stuff.
Check the forums and the issue tracker for many examples of people who make this mistake and were able to correct it by compiling using the right version of Ruby.
Just ran into this post while trying to fix the problem on a recently updated ubuntu 12.10 box.
The solution was to find which version of ruby vim had been compiled against (I preferred to use the vim-nox package and not compile vim from source) - in this case it was ruby 1.9.1. After that I got myself ruby 1.9.1 from rvm, switched to that and compiled command-t. no more crashing.