I know this is a long shot and will probably never happen but, I would love to be able to control, well a few things from my tablet. I would love to be able to connect a tablet so that my keyboard player can have control over his VSTi instrument from a tablet. I have an electronic kit that is triggering Superior Drummer but, it's in another room. I would love to be able to change the snare sound from the drum kit rather than running down the hall and into the control room. It would also be nice if I could have full transport control from the drum kit.
 
I'm not asking for full control of Sonar via a tablet, but just certain aspects of it. I don't need to edit or do complex mixing. I'm fine with not even being able to load the VSTi from the tablet. I'll go to the computer to load it in, I just want control over it remotely when it is loaded. Oh and transport controls. I know I can do this with a wireless keyboard, but its a pain to carry the thing around.

http://hexler.net/software/touchosc - cheap, with some periodic glitches (at least for me), but till you plan to use it live on stage, can do what you want.
https://liine.net/en/products/lemur/ - more expensive, but many users claim it is more stable (and capable).
 
You need to configure both using one or both ways:
1) using related daemon on PC (both apps have such daemon) convert OSC to MIDI, then use MIDI in Sonar with some standard Control Surface (f.e. ACT MIDI) and/or MIDI Learn inside VSTi
2) use my plug-in (AZ Controller, see my signature) to translate commands from app directly into Sonar functionality.
 
(1) is simpler to deploy, (2) has more functionality (f.e. can indicate transport state in applet, display VSTi preset name, etc.) but significantly more complicated at the beginning (see http://www.azslow.com/index.php/topic,295.0.html) and is not "standard" (also free... some people see that a problem )

There are some apps like iDisplay that allow you to use a mobile device as a second touch screen monitor. Though there are issues with android you can have several screens hooked up.

+1 for Android