The folks at hardmac.com have found a way to modify a system extension that came with the 10.6.7 update, to enable trim support on pretty much all SSD drives that support trim.
I tested their modified extension on an OCZ Vertex 2 SSD in a mid 2009 MBP running 10.6.7 and 10.7 and both system info report trim support: Yes.
Here's the link to the article.
As noted by them, the fact that system info reports that trim support is on, doesn't mean it's actually working until another way of checking it is found in OS X.
[crarko adds: This procedure involves editing a kernel extension by hand with a hex editor. I'll put it out here as a possibility if you really think you need it, but if you mess up (especially on a MacBook Air) you may be looking at a wipe/reinstall, so make sure you have a complete backup of the system before attempting this. Proceed at your own risk.]
Mac OS X Hints