I can't imagine why it wouldn't work up at altitude. I want to wait until the reception gets a bit better. I have sirius in my car, with the antenna on the dash, and it cuts in and out a lot. Also, living in the valley, it can be a challenge to find a freq. that isn't already used and often causes static.
I've used my XM MyFi from Acapulco to Alaska in the Learjet at Altitudes around 410. I lost signal the last hour to Anchorage, but you can switch to memory mode for 6 hours of recorded music (fresh everyday). My Copilot uses an IPOD. Inflight entertainment is a must on long trips. I prefer the XM to keep things fresh.
I spoke with both sat radio providers about this and they agreed that as long as the antenna isnt too blocked the reception will be better at altitude. For ideal reception the manufacturers state that there can't be and obstruction within I think 45 degrees of the antenna, but I have installed these on rear decks of cars(inclueding my own) with few problems.
By the way, I have a MiFi for sale on this board. Let me know if you are interested.