I think the tablet idea is plausible. Here's why:
1) As I mentioned in another thread right after WWDC, for the first time in years, there's absolutely nothing in the Apple Store that has me interested. That's bad news for Apple. They need something to tide people over/keep them coming before the Intel switch. People are reluctant to buy Macs now because they're afraid of them being obsolete, blah, blah. However...
2) Many people wouldn't think twice about buying a new iPod. One problem is everyone already has one. Another problem is that you can't do any "computing" with an iPod.
Enter the tablet. It has a pod-ish name to keep people interested. It plays video, if that's your thing. It's cross-platform compatible (files and stuff) and the bundled software is amazing. Non-Mac users weren't afraid of buying an iPod. Non-Mac-using iPod owners will not be afraid to buy this. It will give them a little taste of what OS X is all about.
I think Apple needs a product like this, especially with Palm's new LifeDrive out (a PDA with a 4GB hard drive).
Just my 20 won.
Squire