I've seen two other themes on other sites (xda), but this looks like more work went in to it (i.e. the clock has been redesigned). I haven't rooted my phone either and with cupcake apparently right around the corner I can't say I'm going to do.
If I root my phone now I'll have to update manually and hope everything goes the way it should at which point I doubt very much I'll have root anymore. Or I can wait until someone figures out how update to rc31 and keep root but I know I won't be able to wait.
It's taking a risk but I'll wait until I can downgrade from rc31 and then upgrade to a modded rc31.
The only other thing is that it looks like multi touch is really coming along and I really don't think that t-mobile/htc is ever going to release an official update for multi touch on the g1. Hope I'm making the right play.
Here is some supposedly "inside" info on cupcake:
http://www.googleandblog.com/android-g1-updates-of-mass-cupcakes/3632/