I did a search - only 1 real thread out side of the iPad folks, and in the 1 thread I found the consensus was that working faders in 'Pen Mode' was producing less than desirable results.
I know they are working on a Windows 8 patch for X2, and given the nature of Windows 8, that will probably be a much better starting point for looking to use something like a Microsoft Surface tablet.
Just a thought - I think the Cake site says the patch may be out by the end of the year.
Bob Bone