Several sites describe the process necessary to get TextMate to use either a wrapped (via the rvm wrapper command) or an rvm-specified default Ruby interpreter (including gemsets, basically anything you can use with the rvm use command).
Following just those steps never worked for me, even when I explicitly gave a wrapped TM_RUBY: it would still do an exec ruby in there and blow up. I've solved this (at least temporarily) by adding my ~/.rvm/bin directory to TextMate's PATH shell variable.
