aside from the escalating noise issue, there's another great reason to 'limit' the time (though not the short 30 seconds, for which the escalating noise is probably the best reason) and that is battery drain and loss of images before you save it.
A Film camera barely has any power drain. 1 second to one hour exposure is probably the same power drain on the battery.
D-SLRs, well, we know that's not the case.
Imagine how utterly aggravating it would be to take a 1 hour exposure, and have the battery die at 56 minutes... and nothing saved to the memory card from those 56 minutes. Is an incremental write possible so you don't lose everything from that one long exposure? Possibly... That though would
accelerate the battery drain even more however.
There was a decent article in a British Digital Photography magazine last month on night photography. This reason was given for taking many shorter shots; and taken one
immediately after the other (so you don't have a gap in the star trail) and super imposing them in PS or a grafx program of your choice.