Hi.
An A1200 style case with usb on the back would be really cool, but as a few have pointed out it's not very practical. I think a small box design with wireless kbd and mouse would be better. Maybe something like the CDTV, only smaller and more functional?
An A1200 style case with a few tweaks would probably sell quite well (in Amiga terms), though, and if anyone made it they probably wouldn't regret it.
Some cool mini-itx stuff:
Mini-itx.com