"If the former, and you want touchscreen, you're pretty much out of luck. Sorry."
Not really. I have a SmartQ V7 tablet that triple boots Android, Linux, and Windows CE. They exist but generally aren't great. The V7 has a resistive touchscreen and is a bit old at this stage but does web browsing and HD video fine. I also got Debian up and running on it.
I'd guess Android is the most well known one but that depends on what you consider 'linux'.
I think the difference between tablets and desktops is such that the underlying distro probably doesn't matter that much, it's more down to the frontend (I.E. Gnome looks pretty much the same whatever distro you run it on). I assume Gnome3 would run pretty well on a tablet although it's probably down to personal opinion
If the former, and you want touchscreen, you're pretty much out of luck. Sorry.