In reply to MSchobitz:
The long exposure noise reduction works by a process called dark frame subtraction. This takes a dark photo to work out where the noise is, then removes it from the target image.
It is possible to take dark frames yourself and then use them at your convenience (ISO, exposure time and temp should affect noise).
However, I've found this to be far too much hassle for the benefits it gives.
I'll usually have the long exposure noise reduction off, and once I've done all the test shots I might consider switching it back on for the final shot.