I finally resolved this issue and would like to share what I did in case it helps someone else.
My mobile keys 49 unit stopped working with any application after installing ios7 on my iPhone 4s. I checked to ensure that all of the compatible apps were up to date. Then I tested the keyboard on my computer using some desktop daws, and it worked fine. I stumbled upon the line6 monkey updater software and decided to download it to see if it would resolve my problem. It connected to the keyboard just fine, but didn't indicate that there were any updates available and didn't seem to do anything to the unit. After I disconnected from the computer, I plugged the keyboard into my phone and opened up the line6 midi recorder app. Lo and behold, it instantly told me there was a firmware update! It never lollipoping did this before, so I assume that my connecting the unit to the monkey software triggered something. I have since tested the keyboard in my compatible iOS apps, and it works just fine.