I believe the name will heavily rely on what OS it will run. Apple has been very clear to distinguish the iPhone OS at OS X rather that Mac OS X. If the alleged tablet is indeed an extension of the iPhone platform, the i prefix will probably stick. As someone said, it's too valuable to abandon.
I do feel that it has been all but dropped from the Mac line. The only reason the iMac is remaining is probably due to the iconic status it's reached in the company's history. If it is part of the Mac line, the Mac prefix will probably come.
Also, the 'iSlate' name was trademarked around '05 IIRC. Look how much changed since then that would possibly make such a name irrelevant. They could have been doing it just to have an available option.
Anyway, the slate thing sounds kind of neolithic...slate? Calls to mind dark stone floors. iTab sounds like a web feature, MacTab sounds like a pill. It might probably be something completely unexpected like the MBA, which I still think sounds strange. macpad sounds reasonable, I think. Reminds me of a nice leather tablet.