also, I had managed to find some sort of Internet hub/switch that has RS232 ports and Ethernet too. it was meant to be used the other way (sharing Internet modem connection to computers on LAN) but can work this way too.
PortMasters. We had a few of those connected to 28.8k modems at the ISP where I used to work. Fortunately, we had gone to PRIs before I came on-board, so these were phasing out completely.
The TI forum at AtariAge was recently discussing EIA-232 TCP/IP adapters. Can't remember off the top of my head what they were, but I recall them not being cheap. Think someone's running a BBS on one.