In 10.2.3, Appletalk is its own network port (which goes against my understanding of the term port, as it's really just a protocol over the ethernet port). As such, all three (Modem, Ethernet, and Appletalk) can be used simultaneously and Appletalk / Localtalk devices can be used and auto-detected while also accessing IP devices. Prior to 10.2.3, you had to choose either PPPoE or Appletalk in the Network Preferences, but could not access both over ethernet.
An example of how this can be useful: I've got an old TI printer connected to an Asante Ethernet / Localtalk bridge. My Internet connection is a DSL modem. I run the bridge and modem to a basic ethernet hub and out to my machine. Prior to this, I had to disconnect from my DSL modem to reconfigure my network settings using Location Manager and some scripts to connect to the printer (actually, it would randomly connect to both at times, but never reliably and it would reset on reboot). Now I can just print as you'd expect.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030120065342111