Until AOL releases a Carbonized or Cocoa version of their Mac Software, AOL will only be able to be run from 'classic' in MacOSXb. I'm unsure of how well it will be able to access the modem for it's own connection. However if you access AOL via another internet connection (AOL's 'bring your own access' type account), it should work flawlessly providing MacOSX is setup for the network connection.
Earthlink, just like every other non-proprietary ISP. Basicly everyone that uses PPP type connections will work flawlessly, there have been reports of it actually be noticably faster in OSX.
For those using DSL, Cable Modems, etc, just configuring your network settings as your MacOS9 was setup should work nicely.
~Blake