If the 750 card includes HPC2000 drivers for StrongARM, you should be able to load them on your ADS system and use the card. You can use ActiveSync to load the driver to your device, or you can execute the CAB files directly on your system (loading CAB files directly is quicker during development and testing).
When I last looked at the AirCard 750 (summer 2002), it had Canadian coverage in North America on the Rogers network, but none in the US. We would be unable to test it at our facility.
During your evaluation of the AirCard 750, make sure that the power draw of the 750 doesn't exceed the PCMCIA power ratings for your ADS single-board computer.