Technically, there's no reason why a single radio card couldn't handle multiple network connections at the same time on the same channel. The card might have simulate multiple MAC addresses to avoid confusing an access point, but it's mostly about market positioning. There's been no market to have multi-associative Wi-Fi adapters, and I don't know that someone can just hack the firmware, since Wi-Fi firmware has been the toughest nut to crack. A USB adapter is probably the only real way to do what you want to do on an iBook. Or get a Linksys WET11, which is pretty portable, and can proxy over its MAC address using MAC address translation.