Hi Frank,
Ok. I just opened Textmate and hit File -> New From Template -> Ruby -> Ruby Script. This opened a new file with some predefined commenting at the top. I simply put in puts "Hello World!" to test. I went to bundles -> ruby -> run script and I get a blue box. At the top i still have the missing cat command. Then in a light blue box it says it ran RubyMate r3391 running Ruby v1.8.4 and ran my untitled document. In another light blue box, it says Program Exited normally. At the end it shows the missing rm command error. At the bottome of the document it shows I am using the Ruby language setting.
I'm just guessing here (and sorry if what follows makes you roll your eyeballs with contempt!), but maybe you've reset your PATH variable so as to include /usr/local/bin (and hence the new ruby version), but exclude the usual directories? Check any PATH setting line in your bash startup files (.bashrc, .bash_login, or .profile); make sure it's not setting the PATH to simply /usr/local/bin, but instead reads something like
export PATH="/usr/local/bin:${PATH}"
Best of luck, Paul