It is, considering that you're talking about centimetre-band frequencies, so unless the unit was specifically designed with high-speed applications in mind, it will in all likelihood never find the signal as it will be outside its scan range.
LH2, can you expand on this a bit more? With my basic high-school physics it would seem to me that the handheld units and the satellites have to contend with a Doppler shift resulting from speeds between -15K and +15K knots anyway - after all the satellite is moving towards you, or overhead, or moving away from you at (very) approximately that speed. Why is 150 knots extra in either direction going to make a difference?
Also, the Wikipedia article on Iridium specifically talks about frequency bands separated by idle bands to allow for the Doppler shift.