Hiptop3.com seems to have the juice on the forthcoming T-Mobile Sidekick Gekko, also possibly known as "Aspen" and codename: Style2. Looks like Gekko will take the place of the now-defunct Sidekick ID at the low-end of the SK lineup, and will feature some kind of color-changeable front faceplates (hence the moniker) and other customizations.
Gekko may also use the same Sharp LCD display currently found on Sidekick LX, and a feature set to match the LX, or close to it. Odds are Gekko will keep iD's larger, more cheaply made body in order to differentiate it from the smaller, sleeker LX in TMo's lineup.
Tmonews.com is going so far as to pencil in July 27th for Gekko's launch. So, you know, don't buy a new Sidekick before then ... if you can wait.