SLIP is stable at 19200 on most machines, some users with really fast ZIPs have reported using 38400 with no problems. CSLIP is not (yet) supported.
Dial up is currently supported by calling a third-party product installed in /usr/sbin/slipdial to establish the connection. Use of a dialer like James Brookes' slipdial will allow you to use a full featured scripting language for automatic dialing and provide support for dynamic SLIP addressing.
MacIP also uses a CDEV to configure itself, installation only requires copying the CDEV into your */system/cdevs folder and re-booting.
Currently this interface only supports static address assignment. Server assigned addressing will be available in the near future.
I have an initial port of PPP running based upon the public domain ppp-2.1.2b release. It is a full featured port including CHAP and UPAP authentication. I still need to add VJ TCP compression and re-write the low level routines in assembly for speed, as well as distribute the code to a few select beta testers before it will be ready.
I hope to be able to also provide a communications path for the built-in AppleTalk stack via PPP. Many terminal servers allow AppleTalk packets to be sent via PPP; the University of Illinois for example provides this.
I have developed an ISA interface for the Apple IIgs that plugs into the processor slot much like a TWGS or ZIP. All this stuff working together allows me to use ethernet with my GS. Unfortunately, the current version of my ISA interface doesn't work with a TWGS due to problems with the RDY line (I haven't tested it with a ZIP). If I ever manage to get it working with a TWGS or a ZIP, I'll consider putting it into production.