I'm pretty sure the OP meant that they were going to use the Treo "in flight", since that's included in the title of this thread. I don't think they are attempting to use it during takeoff/landing.

I would think "flight mode" is appropriate for most airlines. I think ATA is the only exception I've heard. I have used my Blackberry in airplane mode, while in flight, on both UA and AA recently with no problems.