if the modem is external and connects thru a serial port (mac serial ports are a lot different then Linux ports) then you can be sure that it will work.
The way hardware modems work is that they have their own little procccessor their own little firmware thing that works like a bit like a simple OS. Then Linux or OS 9 or Windows just simply talks to the modem thru a series of standardized AT commands. Stuff like 'ATAT' or 'ATDT', and then modem responds. It's pretty much like 2 machines talking to each other thru the serial port.
Now a serial port can easily be done thru a PCI or a ISA port, but almost all internal modems, with very few excepts, are software, or partially software (aka winmodem). In this situation the modem is mostly emulated in software and what you end up with is a card that is basicly just a digital to analog conversion device.
Trouble is these cards are propriatory and secret and they won't tell any Linux developers how they work. Some have been reverse engineered, some haven't.
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