Forgive my ignorance but do all such tablet devices require pen input only? I'd like to try one such interface in a Max DMI, but using fingertip interaction (not necessarily multitouch); should i be looking at, for example, the HP Slate or other iPad-alikes?
The iPad will definitely do what you want and there a number of apps with Max patches already available or easily programmed to do just about anything you want. Examples are: TouchOSC, Mrmr, Fantastick, and c74. Each has a slightly different twist on how get things done. Mrmr is free and pretty well supported, and TouchOSC while not free is very nice and a native iPad app.