17,500
I think there is an Apple published specification about using the GPS above 10,000 feet, and once heard that the limit was a certification issue. I'd like to know more.
I can say that my ipad1 always holds signal and performs normally up to 17,500 MSL. (I've flown paperless for since March 2009, many trips at 15 to 17.5 for winds. The ipad is not primary GPS, but it is the only "sectional chart/WAC" graphic onboard.) Lots of long trips, no signal loss. Never.
For other common concerns:
1. Heat. I did once get an ipad shutdown of about 4 minutes when the ipad overheated sitting on the glareshield. Not a big deal with other gps/nav capability, but it was troubling. The ipad gives no warning of impending heat related shutdown. It just shuts down. Clue: don't put ipad or iphone in direct sun for long.
2. GPS signal lock. Never had signal loss in an RV. Canopy provides a clear shot to satellites. The overhead metal in a C-172 class airplane will interfere and the user will be jockeying the ipad to retain uninterrupted signal. My conclusion: a separate GPS (wired or bluetooth) is needed for metal roof planes or non-3G ipads only. It adds nothing for 3G ipad running in the RV environment. (Caveat: the RV10 might be different.)
Related issue. If you shut the ipad down, it will take longer to "find" location once in the air. That is what the "assisted" GPS deal is: quick orientation upon startup using cell signal to obtain general location. If you start the GPS after takeoff and climb (which I did a time or two), or restart it while in the air after shutting it down and changing location substantially, it will take a while to get back onto satellite lock and a good fix. I don't remember the time, but it seemed long, perhaps 5 to 7 minutes. This is probably not the case if you are close to cell signals, say 3000ft agl or less. But, at 8K AGL or more, cell signal is not available (distance and metal beneath you) and the ipad takes a while to find signal without that cell tower assistance. Conclusion: start your nav program before takeoff.
3. Battery. GPS operation drains the battery faster. Starting with a 90 to 100% battery, I am down to <20% sometimes after 4 to 5 hours enroute. My ipad1 has a shorter battery life than the ipad2, apparently. So, I plug into power in the cockpit. Beats trying to get a charge at an FBO between flight legs.
4. Screen: Usable at all times, but sometimes needs trial and error of a couple angles to get readable in bright sun. Maybe the new screens are or will be better. Does not compete with Garmin, Vertical Power, or Advanced Flight screens -- but that should not surprise us.
Hope this is helpful.