jonor: actually, there is:
it already more or less works. however, it's initially targeted more at ARM-based things like the SheevaPlug and various dev boards that at phones/tablets per se. Significantly we don't have shippable accelerated drivers for any ARM graphics hardware yet, though that should change when the new OLPC hardware comes along.
Notwithstanding, there are some Intel-based tablets around:
is the most commonly used in the very small niche market of 'people futzing around with traditional Linux distros on tablets', but there's also Archos' 9, Acer's one, and a couple of others, I think. They're not exactly highly desirable hardware - they tend to be somewhat on the large side (11" screens) and pretty heavy - but they are actual tablets you can actually put a Linux system onto. I think at least some of them use the infamous GMA 500 graphics chip, which can really put a dent in your day, though.
I'd say right now GNOME 3's main interface is rather nice on a tablet, but - and it's a big but - there aren't really a lot of apps you could run on top of it, on a traditional distro like Fedora, which would be much fun to use at all. You'd still be stuck trying to do your web browsing with desktop Firefox and your mail in Evolution or something, which is no-one's idea of a great time on a tablet. GNOME does seem to be working on revamps to the UIs of various apps which would be more tablet friendly, though, at which point things would likely get more interesting.