Really I think this is just irrational, just because it will run an OS derived from iPhone doesn't imply anything about it's final capabilities. On top of that I've yet to see anybody explain to me how one would get Mac OS/X software to work usefully on a tablet. If the screen is a Touch input device Apple doesn't have any options other than iPhone OS.
The fact is the tablet will need a reason for it's existance beyound running a desktop OS because history here is starkly clear. Running a desktop user interface on a tablet has always been a huge failure. The reason is pretty clear too, you can't input data 8 hours a day on them. For that matter 15 minutes at a time is a joke.
The fact is apps need to be refactored for use on a tablet. Going to a new API forces that. Fresh apps are guarranteed with iPhone s as the programming API.
It is notable I'm talking about APIs here. Underneath iPhone OS still runs the same basic kernel. IPhone OS is extremely capable even with the acknowledged limitations in the recent distributions. We don't know though how those limitations will reveal themselves in an iPhone release on a tablet. I would imagine though the limitation on multitasking would go away in some way. Multitasking would be a huge improvement and is just one limitation that a more powerful tablet could enable.
The key here is that we don't know what Apples goals are here at all. They could just throw 256Meg of ram in the thing to barely get by doing movies and magazines. This to keep the tablet dirt cheap. Apple could deliver a serverly constrained tablet for under $400 to meet the needs of this target use. Now I'd rather see 2GB of RAM, physical interfaces to the world, advanced GPU and display tech, dual CPUs and a host of other goodies. That won't be a $400 dollar machine though. Either way the only rational OS is something derived from iPhone OS.