The Atari / TOS (and even Atari 8-bit) and Acorn / RiscOS groups are still quite active.
A new announcement I noticed on the Atari 8-bit scene this month was a
new graphics card for the Atari XE - it plugs into the socket where the original GTIA chip was. Hi-res modes, 256-colour hardware sprites, 65536 colour palette, Quite amazing that developments such as this are occurring in 2006 for an 8-bit computer whose chipset was designed in 1978 (by Jay Miner while he was at Atari).
One big difference I have noticed is that they seem quite ready to admit their machines are not going to have a 'comeback' but are happy to keep using them, upgrading them, and developing for them simply because they enjoy it and they like the computers / OS.
It just seems to be certain people (albeit a minority these days) of the Amiga community who for some reason need to believe that Amiga will miraculously make a comeback, and are oblivious to all logic as to why this won't happen.