You can run themes on the latest firmware, you just lose some of the pieces like the png transparency etc which I am sure someone else can explain a lot better than I. I am personally running mojo on 3.01.09 and very happy with it. I get a pretty decent theme that functions well, and I also get all the new features of the 3x firmware series which I would not want to be without.
I have several computers, a hub and a liveplus which all have movies, music and pictures on them. The below capability alone makes the HUB so much better I cannot imagine not moving to the 3x firmware
- Supports adding network share folders into "My Media Library" (Use options button).
Info on the cpabilities of the latest FW here
Good luck on your choice, but in my view you can have the best of both worlds if you are willing to get a little less out of your theme. The upside of 3.01.09 is just to great for me to be stuck in the 2x FW versions.