Daniel,
I believe the USB ports on the ASA5510 cannot be used for anything yet, (i.e. USB to PCMCIA adapter)
I know it's an additional piece of hardware, but perhaps you should consider using a device from CradlePoint, like the MBR1200, or CBR 450 (USB or ExpressCard dongles only) to provide a wireless connection to your backup ISP, and plugging the cradlepoint device into the ASA?
http://www.cradlepoint.com/products/branch-office-retail-pos/mbr1200-failsafe-gigabit-n-router-mobile-broadband
Then you could configure the ASA to use the cradlepoint connection as a redundant ISP connection:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00806e880b.shtml
I hope this helps,
- Ken