I would never consider working on a tablet, even if it could perfectly run my development environment. I absolutely need a keyboard, and I might as well take the laptop if I'm going to pack a keyboard with the tablet.
Agreed, it's not only the keyboard but also the blazing fast application switching (at a keyboard shortcut distance), decent code editors.
The only thing that come near that for me are the asus transformer series running ubuntu, but I'm still encountering multiple fatal issues (plus the touchpad is sub-par).
Also the storage space. Tablets and ultrabooks still don't give you enough space if you need to install a lot of development software, database servers, etc.
Oh, and if you need to develop in more than one OS, then a tablet isn't going to cut it, and the 128/256GB SSD in an ultrabook is a pretty tight squeeze for dual boot.
Those factors, the RAM and the back pain are the reasons I stick with larger devices for now. I do have my beady eye on the forthcoming Asus U500 though.
