I guess the 1Ds4 rumor is really running wild.
Don't know what those guys at Canon are thinking about, but I think there's several points we should watch regarding the 1Ds4 and 5D3 situation.
1. 5D vs. 1Ds: I think the problem is pretty obvious with the backlash with the 5D2 release from 1Ds3 users. You got a affordable FF (albeit not as good in AF, FPS, weather-seal, etc.) within a short time after the release of the professional camera. Given this escapade, would Canon do the same thing again, or will they have some sure mechansim in place to ensure this does not happen again (like drag out the time/cycle between the 2)?
2. 5D3 vs. D800: Yeah, we all know the D800 should be out sometimes this year. While Nikonites might disagree, the 5D2 definitely took a significant bite in the Megapixel race and video capabilities. Since 5D2 has the lead, I would assume Canon would tend toward waiting and to let D800 make the first move (unless they're so confident with their product that they believe releasing it early on would leave their competitors way behind). Of course, there's also the D4 vs. 1Ds4 flagship model competition... though I think that has less of a impact on the mass consumer market.
Just my 2 cents...