Thank you all for your replies.

I didn't think I needed to consider Miami/Roadshow TCP-IP stuff. I don't want to do dial-up internet (do ISPs still offer that?) - just connect to dial-up BBSes.

If you use a program like NComm, or BBS software like Max's BBS, you point the software to the serial device, and that then sends data directly to/from serial. You can generally change the settings in these programs so they use something other than serial.device - e.g. if you had a Zorro multi-serial port card which might have its own serial device.

I was thinking that Poseidon would, when happily linked to a USB modem or a PL2303 adaptor, funnel the data to serial.device or an equivalent, that a serial program could then use - happily oblivious to the USB aspect behind the scenes.