Another kb article adds NAT-PMP for the ISP router/bridge - and says something very stupid:
The "Note" is the stupid part. Does Apple think our homes have an ethernet connection to the net?
... You may need to manually enable UPnP or NAT-PMP on your routers. You might also need to contact your ISP (Internet Service Provider...
Note: Apple AirPort Base Stations are the preferred method for accessing the full potential of your Mac and iLife apps. If configured properly, they should work seamlessly with Back to My Mac.
I won't install Leopard until 10.5.3, so I'm not following the BTMM discussions too closely. I get the impression Apple hacked the security model of OS X to enable BTMM, that makes me wonder how well it works with multi-user configurations. What if another account is running on the Mac I want to get "back to"?