I hate to say it .. but please check the other discussions; for one, this has come up on pretty much every console for the last decade, and for the other, we've discussed it to death on the gp32, ghe gp2x, and are currently discussing it in a few threads for the pandora

Its not a new idea, it always has been an interesting idea, and it is often full of pitfalls (especially for machines that have a complex set of options, such as an Atari 800, Amiga, ST, DOS machine, and so forth.)
In the end, if someone made a simple API that managed all the GUI elements, so porters/coders could just adopt it, it'd probably take off. But the peopel who are desired to use such a thing, are busy working on their ports

jeff
Edit; sorry if that sounds cranky .., I've not had a good sleep in over 19 months so I am a little cranky sometimes
To wit ..
Firing up an emu with a ROM-path (say) may work well, assuming they all honour a simple common command-line (say), or at least the front end has a config system sufficient to detail the launch of each app. Showing screenshots and such for the roms is hard (mapping them back to useful pre-screenshots, or at least a standard for the emus to export screenshots the front end can use.. form savestates or whatever). The larger problem is for any machine with any form of configuration .. while in the past we had performance issues so SNES may or may not need transparencies on and off and so on, the panda should be able to handle much of the old school emus no problem. That issue still applies to newer machines probably, so the front end shoudl need to handle a lot of emu- and platform- specific options. ie: Some PSX emus may offer some options that others do not, say. So lots of command line settings reqwuired in the emus, and a healthy config system in the front end to support setting all these options on a per-game basis. (And having a dsimple front end is soert od defeated in purpose if oyu have to configure the hell out of it sometimes, when its pretty easy for emu authors to just support options etting in their own app, all fresh and custom.) Other problems come into play when you need to configure how much RAM, what kind of audio, what sort of hard drives, where al these things mount to (consider DOSBox), and things like which chips to enable in the Amniga emulation, to what drives to mount as virtula hard disks on the ST, which multiple floppies to support. And then things like supporting multi-disk games in an ST or Amiga or the like ..
It gets complex fast for more complex systems.
A simple common front end itself works well for simple emus, always has (see thigns on DC, Xbox, etc.)
A simple common front-end API can work well, but its a lot fo work to set it up, and it needs to be very general. By the time its ready, the emus are already well on their way with cvustom UIs, as it is very easy to slap a custom UI together. (Consider.. porters and coders are more interested in getting their app up and going and working on efficiency and so on .. they're not spending all their time on fancy front ends, until later.)
A common set of APIs to make USB-controllers and so on would be nice too so I say work on that

jeffcrank