Did a little experimenting with night exposures using the 645D...my verdict is that if you're serious about night photography, i.e. star trails, really long exposures, then you're better off with one of the 35mm DSLR's, though in a pinch the 645D can be workable. First, you have to factor in that however long your exposure is, you will also be doing a dark frame immediately afterwards, whether you like it or not. So...by the end of a long day of shooting I only had the stamina for one 30 min exposure, then left the camera on and went to sleep for the night! I was pleasantly surprised by the results in the AM when I took a peek. LOTS of noise, which I was able to mostly correct in PS, plus some other spots that came from who knows where...I cloned those out.
Pentax 645D, 35mm A, approx 30 min exposure, ISO 800, f/3.5