TRIM is a feature in solid state drives to handle some garbage collection duties that keep the drive performing better. More specifically, TRIM allows the operating system to inform its SSD which blocks were no longer needed and can therefore be erased by the drive itself, keeping everything tidy in its little solid state brain.
TRIM had been enabled in the MacBook Pros that shipped in February through a version of Mac OS X 10.6.6 specific to those models. It has also been working in developer builds of OS X 10.7 Lion for some time, but as noted above, Apple had not announced that it would enable to feature in Snow Leopard.
The image below was posted by a MacRumors reader showing where TRIM was enabled.