It all depends on what operating system it runs in my opinion. Most tablet operating systems are just glorified, "user-friendly" cellphone-based systems (Android, iOS, WP7), and should not be on Super User in my opinion. If the tablet runs Linux/OSX/Windows 7 or 8/Unix, then it's fair game.
Why? Well, can you create and program an application while running the device? Do you have the flexibility and choice of compiling your own applications that run on other operating systems?
I'm not saying that you can't do these things, but use some common sense. There's so many applications I use on a daily basis that work on Windows/Linux/Mac. And if it didn't, I could (in most cases) easily just compile it on the new operating system.
The same can not be said for most phone/tablet operating systems, and I think that's where we should draw the line. These are highly embedded devices with very specific hardware and software requirements, removing most of the modularity from the system - and that's what a "computer" is all about. I think KronoS worded this point very well:
One thing I would add is the fact that the hardware cannot be upgraded
either. I.e. The memory cannot be raised. The fact that it's an
'embedded system' makes it just like the game console and the cell
phone. I forsee SE starting it's own dedicated site for these but we
don't need to service them.
My final argument, from the FAQ, Super User is not about electronic devices, media players, cell phones or smart phones, except insofar as they interface with your computer. If you want to argue that your tablet is more then a smart phone, we'd love to see some evidence. There is a distinction here, and you can't ignore it just because you feel like your question fits here.
If your tablet runs iOS/Android/WP7, it does not belong here. If it runs Windows 7/Linux/OS X, then it belongs here. 'Nuff said.