You can write USB serial device software (we've done it) which requires no drivers for Mac, Linux and Windows (except the INF file). The CDC/ACM model is what we used. However, it was a real pain. Windows requires a .INF file whose rules are obscure, especially for Vista. If you make a composite device, e.g. serial and mass storage, Macs don't understand the IAD descriptor and you need a kluge which both Windows and Linux accept.
Stephen