As I mentioned earlier in another area perhaps,
FW 800 includes the FW 400 spec,
IOW, if one has a FW 800 port only (the new MBP), this is not an issue at all.
FW 400 devices can plug into FW 800 ports, with the right cables
FW 800 to FW 400 6 pin
FW 800 to FW 400 4 pin (video capture)
Yep, it'll be just like USB/USB2 and all its port shapes.
Either use adapters for your cables or simply buying a 9-pin to a 4-or-6-pin cable to make things easier.