As you probably know, the current Mobile In product info says this:

Works with iPhone® 4S, iPhone® 4, iPad® (3rd generation), iPad® 2 and iPad®

Line 6 typically does not provide advance info on pending product upgrades. Clearly this particular situation is complicated by the introduction of a new h/w interface with the iPhone 5. Unfortunately, I think we'll just have to wait and see. We'll know it's compatible if and when Line 6 makes an official announcement, and probably not before.

As a general rule I believe one should buy any product based on what it can do today, not on what it might be able to do sometime in the future. Based on that, if  the Mobile In provides sufficeient value to you with your iPad, then purchase it. Don't purchase it if you perceive sufficient value only in its ability to work with iPhone 5.

I have the iPhone 5 and connect mobile in via Apple's 30 pin to lightning adapter. Works flawlessly.

One thing to note, the 30 pin to lightning adapter can fit into the iPhone 5 in any direction. (Unlike the old 30 pin adapter that only fits one way) I mention this because once you plug the mobile in adapter in, it may block the iPhones headphone jack. Solution is simply unplug from iPhone and flip Apples adapter the other way around.