The Gallery theme by Devicious seems to be the first and only theme at the moment that works with the latest FW.
Rolling back to 2.07? The only thing i can think of is you lose some of the internet apps, the RSS function and i'm not sure about the USB hub support.
Well, losing the ability to aggragate various servers / directories through "My Media Library" is a big loss of functionality if you are staying on the 2.x FW. I have movies and music on three different machines and they all show up in one easy to find place. That's a huge coup that I wouldn't be willing to give up.
Also, Mojo works pretty well on the 3.x Firmware. I have it showing the thumbnails, displaying the metadata from XML that I get from HubGen and backdrops from the movie. I am honestly not sure what you dont get out of the theme with the upgrade, but I certainly wouldn't say that it does it does not work on 3.x. I am extremely happy with it.