All the pro's (Larofet and similar) say that you can run a 7D to 800 ISO without much grain, a 5D MkII to 1600 ISO and a 1D MkIV to 3200 (that's why it's $5k). They also say, and I agree, that if you have to shoot wide open (night time) and still can shoot in 800 or less ISO, go with a 7D. Like chillipie said, the 5D sensor is really hard to keep focus on especially when you're running a fast prime wide open.
Look at the episode of House they shot on the 5D. These guys are pros and the shots were constantly in and out of focus.
A good cinema lens (like the Zeiss compact primes) helps to follow focus because it has constant interval marks on the lens. That means if you turn the focus ring a half inch you change focus 3 feet, if yo doit again it's another 3 feet, etc. On photo lenses, that's not the case. The first inch might be 3-5 feet, the next inch 5-10 and the last 10-Infiniti. The compact primes by Zeiss are the cheapest cinema lenses you can get new, and they're about $3700 a pop.
In daylight, most people still prefer the 5D because you can stop down the lens and still maintain a shallow DOF. On my 7D, in daylight if you wan shallow DOF you have to go open (or close) and stack ND filters on to fight the sun. Nobody likes the 1D in daylight, it's too expensive then and it's color is a bit funny. Again, for night time it's the best DSLR there is. It can actually see more into shadows than the naked eye.