as mentioned, there are usb/audio devices, plus pci cards, firewire devices, and probably others.
it seems to me that apple, knowing its machines are used for pro audio, decided to let people buy (the much higher quality) outboard gear. i'm sure it saved them a few bucks here and there in doing that, for those who have no use for audio in.
that leaves only the hobbyist who would like some decent audio, but doesn't want to spend any extra $$. looks like you're out in the cold (you cheapos
personally, i have a digidesign box that comes w/ a pci card. i've never used my audio in jack -- why would i? the quality stinks.
for the original poster: many of these devices come w/ their own software.
another thought: now that apple has acquired emagic, perhaps an iApp based on it is coming? (i.e. the iApp is simple and free, Emagic's Logic is still a high-end, pay-for item)