There's been a massive debate in that thread? Wish it wasn't over 10 pages, makes it a pain to keep up with. I stopped reading after 3, hopefully the mods will allow multiple threads again soon, since also big thread are a pain in the search results.
Anyhow, I've used my modem in my Powerbook once, only to test an ISPs PortMaster I helped config. Didn't work anyhow, as my only phone line in the house is Vonage, so I ended up using someone elses laptop at their house.
FW800, never used it. I have a FW400 enclosure, and an iSight, but never needed to use both at the same time to justify buying a FW800-FW400 cable. If Apple would have thrown in a FW800-FW400 plug adapter, it might have been used once or twice.
The speed gains on the new system are much more important to me then losing a firewire port, or having to get a small USB modem. I doubt the modem will ever return to any Apple system in an internal way, and FW800 might not, so I suppose if these features manage to outweigh the performance gains, then grab a PowerBook G4 now.