Pardon if this is a dumb question, but does this thing look like it could decode and play MP3 files? With a little power left over for a simple user interface? I'm interesting in making a specialized MP3 player of sorts.
This is entirely the wrong product for that. The SoC on the Raspberry Pi is a somewhat high-powered graphics processor; if all you need is MP3 decoding, your needs will be much better met by a mid-range microcontroller. There's even ASICs (like the VS1011) that'll do most of the work for you.