As someone who is running simply a large trunk with LEGO parts where a new NXT kit gets emptied out to for further building I have found it hard in the past to answer frequent questions like
"Can this model of your's been built with a single retail kit?"- figuring out the answer to that was somewhat troublesome and required a lot of comparing to the according parts inventory at Peeron.
Hence I finally implemented a Java application that may come in handy: Model Differ (working title).
It allows for comparison of two models that are laid down in LDRAW format and lists the parts that are mutually not contained (honoring also the number of occurrences of a particular part). For convenience, it's possible also to check a model directly against the NXT Retail Kit.
If you are into CADing your models with LDRAW, have a look - the only thing you need is a Java Runtime (version 5 or above). And some LDRAW model, of course.