I saw this at TUAW and thought it was quite interesting. Not just from the point of view of having a multi-touch enabled case for a device like an iPad, but also in terms of what additional benefits it would bring to musical performance.
On of the issues with a device like the iPad in terms of performance is that you have to hold it to use it or put it down flat which is never as good in a performance (IMHO).
If you the casing itself was multitouch enabled then you could in theory have even more control over a performance based app even with the hand you were using to hold the device.
I think that could be really useful.