I'd been looking at one of these for a new live-set machine. Sounds rad, right? I got my midibox, then would just make another small box with mac mini, audio interface, a cooling fan, then get a cheap small display and mini keyboard/trackpad thing and you've got a sweet live rig that's still all small but not vulnerable to spilled beers and clumsy louts like a laptop is. worst case you wreck your $10 USB keyboard...
The killer here is the power supply issue...I guess you could use a UPS live in case of drops, but in my experience it's important to be able to set up FAST, which usually means having the laptop in sleep mode, flipping the lid, pluggin in and being ready to roll. If you had to boot up every time it would suck...do ya think there's some way to rig a battery to allow the Mini to sleep for a few hours while being shuffled around? That would totally clinch it for me.
As a studio machine it seems cool as well, and I imagine the big-daddy chips will one day make their way into these things.