"I think it's the technology of the future," comments Jim Hamm about SSD, as he sends us this article from Small Dog Tech Tails. Scroll down to "Just a TRIM." It explains why a Solid State Drive (SSD) needs a TRIM (or a similar function) to prevent the degradation, over time, of the write function. (TRIM, while capitalized, is not an acronym for anything; its purpose is to trim old data from the SSD.)
I asked Jim about replacing an existing drive in a Mac with a SSD. He says it's not commonplace, but several new Macs, including the new MacBook Pros with SSD come with Thunderbolt and do support TRIM. He thinks SSDs are available to be used as an exterior drive of Time Machine, and suggests this info from NewEgg.