(Posters should keep in mind that there's a whole family of Momentus drives, but only the Momentus XT is the "hybrid" unit.)
Right now, SD26 is the latest and is available only pre-installed. SD25 is the latest available for download and installation at this time.
Question: Why are you looking for an update? Is there some malfunction or misbehavior?
FWIW, I'm using SD25 under Lion without any trouble at all and am amazed by the speed, even though I have the new FileVault 2 whole-disk encryption turned on. The same drive with the same firmware was unacceptable in a variety of ways under Snow Leopard.