The guys over at EVDOinfo.com have discovered that current Intel Macs already support 3G EVDO cards. Basically what they did is rip the PCI Express Mini Airport wireless card out of a Mac mini and replace it with a previously activated EVDO PCI Express Mini card taken from a Novatel U720 USB EVDO modem. After installing the recent WWAN update (by bypassing the hardware check), the Mac mini recognized the card and established a connection to the Sprint network. They note that this makes the Mac mini an absolutely perfect carputer (except for some antenna issues).
They speculate that this should work on the MacBook (Pro) as well, but obviously nobody wants to lose Airport connectivity to gain WWAN support. However, this does seem to show that there's no technical limitation to adding EVDO connectivity to next generation Mac portables.