Certain airlines say you can use them, some don't. Look in the manual, you don't want to get an already grumpy flight atendant even more grumpy.
The problem with reception doesn't have to do with altitude or speed.* It's the fact that you're in a metal tube that blocks most RF. That's why you see all the antennas hanging off the plane.** Try holding it close to the window and see if it can finally pick it up.
* Tomtom's are really designed for the road and seem to make certain assumptions that you're on or close to a road. Maybe this would make it harder to get sat lock. More traditional GPS systems don't care.
** Find a fiberglass plane, they frequently embed antena's into the structure and it works really well.