My experience working with UG and GM suppliers is that UG's parasolid version is actually BEHIND SW's! We've had to dail-back the parasolid version on our exports for compatibility w/ our customers.
True, SW reads out only the parasolid data from UG files, but if NX3 has a new file structure, SW won't be able to find it. Also, it's possible that there just is no importable geometry available in the UG file (i.e. an assembly sent without components). UG file types do not distinguish between assemblies, drawings, and parts. It is possible to model a part, throw in a component, and create a drawing of that part-assembly all in the same file.
An interesting thing about importing UG files is that SW imports (and suppresses) all of the "tool solids" used to build the model. This makes for some cleanup chores, but also gives some insight into the CAD jockey's technique.
There is a pay-per-use translator called PS-Exchange available at . It allows you to view files for free. Very handy tool.