Ok, my thoughts.
An RM3 module from rogue robotics could play the MP3s for you and control a SDHC card (32GB) for file storage.
For touchscreens etc maybe look at: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1256896315/all but I doubt you can control audio as well without a bit of modification. The RMP3 requires 2 serial pins I think so if there are two pins to spare on the touch screen then you could probably manage that.
it's USB tablet and Arduino can not be a host for USB devices.
Cam / webcam - no way
Sure about that?
http://www.circuitsathome.com/
People have also managed to save from VGA camera modules to SD card and then presumably you could read those images using the LCD shield I linked...
I specialize in PC hardware, and never really ventured out passed that, but I love learning about this kind of stuff. Any links, comments, concerns, pointers, and anything else I would be most greatful for.
There are lots of tutorials if you search for them. There are so many I would not like to recommend one.
You would probably run out of space on a 328 (duemilanove) so maybe you want to look at the arduino mega (or similar boards - or make your own!).
I think that doing all of this together with an arduino would be pretty much impossible. The parts on their own should be possible though. You probably want to start off with a simpler project and then maybe move on to an ARM processor or something if you really want to try and do this!
Mowcius