The EOS 5D was announced in August 2005. 5D2 was announced in September 2008. This product line is on a longer cycle than the lower models, but 3 years seems to be about right. That would put a replacement at aug/sept 2011.
Ultimately all of these things get replaced, it's a matter of when not if. Currently the 5D2 seems to be selling well to both the pro video group as well as to photographers. Canon needn't feel rushed to bring out a replacement (since the majority of 5D2 and D700 buyers are already tied into canon or nikon lens systems, so the D700 isn't really taking sales away, and Sony seem equivocal about full-frame support). I suspect that canon will take the time required to make sure that the next camera is a knockout - expect faster CMOS readout (to reduce rolling shutter "Jello" in video), higher framerate (probably 5FPS), 7D AF system and the usual excellent sensor (not sure whether they'll bother to increase resolution, but hopefully a bit lower noise and better DR).