I have
osascript -e 'tell application "R" to activate' -e "tell application "R" to cmd "${TM_SELECTED_TEXT//"/\"}"" &
in my local bundle (note the ampersand which will at least prevent textmate from locking up by running the applescript in the background)
As Jeff mentioned, upgrading your R may also help (I have since stopped using the R GUI so I can't confirm this myself, although I do remember a few related messages on R-SIG-MAC)
It's nice to know that there are some fellow R users using textmate.
Hadley