There have been rumours of Apple's Tablet PC for a long long time. The latest report (at respectable Ars Technica) suggests next Spring. They might wait till WWDC, though they might release it earlier to promote sales. I think the reason it's taken so long to come out has to do with the technology. It has to have multi-touch capability (Stantum gives us an idea of the largest multitouch panels). It has to be able to do everything that the Macbook Air can do. And it has to be good enough for Perfectionist Steve. The drip-feed of rumours about it's release guarantees Apple more fanatical following. I can't wait!
PS 14 Dec: The rumour mill is going into overdrive: TheRegister quote from a French site that shows a video of the Tablet in action.
PS 5 Jan: Google are also apparently working on a Tablet, whilst Pravda (Russia's most famous newspaper) reveals the Apple Tablet will probably retail for under $999 from Easter.