Perhaps some of the later 2009 model SSDs would work with TRIM. I had a Rev B with the SSD, and when I switched to the Rev D last year, I noticed a BIG performance difference. SSD technology changed rapidly between 2008 and 2010. It didn't become standard on the MacBook Air until 2010, so that may be why there is confusion as to which SSDs are compatible.
I would believe this might have something to do with the older SSDs being PATA and the newer SSDs being SATA. The Toshiba SSDs that are used in the MacBook Pros all support TRIM. The SSDs in the iMacs (BTO option) are Toshiba and support TRIM as well. The Blade SSDs in the Rev D and the new 2011 are also Toshibas and SATA-based and support TRIM.
The older SSDs used in the RevB and RevC MacBook Airs were NOT Toshiba and they were NOT SATA II as I understand, and this leads me to believe that this is the reason they don't support TRIM.