I think MacPad works the best. It keeps the two syllable trend (MacBook, iMac, Mac Pro, iPod, iPhone), and then you could have MacPad Pro, MacPad Mini, etc. Any thing with "i" in front sounds pretentious, and words like slate, glass, paper and tablet are either too vague or are awkward to say.
MacPad and MacPad Pro. Easy and fun to say, just like they should be to use.