most DSLR's dont record more than 29 mins and 59 secs, if its 30 mins or up its classed as a video camera is they have higher tax prices (iirc)
Almost all DSLR's suffer with rolling shutter, even 5D's. 7D's etc. Its normal.
the 550d / 600d will give the best DSLR video quality for the price, better than the d5100. But how much better is your call. To me its enough to warrant going for the 550d/600d (they are almost identical), but i do video work and obsess about the last little bit of sharpness and resolution.
The d5100 has autofocus in video, which isnt vital for pro video, because thats normally manual focus, but for all other purposes its fun and useful, quite fast too.
If video quality and sharpness is your priority, the Lumix GH1 is the best bet. Get it hacked and it has much better sharpness and resolution than even a 7D.
but the GH1 isnt a DSLR, its a micro four thirds camera, much like my GF1.
You have fewer lenses and accessories as its a newer system, but its more compact and takes excellent video.
2nd the GH1 with custom it easily beats a lot of the hd camcorders.