It's not just in Lion: new MacBooks Pro with SSD have TRIM support already in Snow Leopard

Mac OS X 10.6.6 build 10J3210Apparently, only Mac OS X Lion that will gain support for TRIM: since the new MacBooks Pro that are leaving the factory with SSDs have this feature, even running Snow Leopard, according to the AppleInsider.

This is proved by the fact that these machines run the build 10J3210 instead of 10J567, which corresponds to Mac OS X 10.6.6 publicly available.

It is possible that a future update will expand the availability of TRIM support for other models, but for now it would only be available even for the new MacBooks Pro with SSDs configured at the time of purchase (ie, later installations would still not have this benefit).