Page 62 of the owners manual:
With AirPort Extreme technology, your MacBook Pro makes a wireless connection to an
AirPort Express, Airport Extreme Base Station, or any 802.11a-, 802.11b-, or 802.11g-
compliant product that is connected to the phone line, a DSL or cable modem, or a
local-area network with Internet access.
I'm assuming as long as a 3rd party card uses the same Atheros chipset that the MBP uses, the Airport driver might work with it for 802.11a, similar to how some PCMCIA cards just worked with the Airport driver for 802.11b.