I have 5.0.1.18 and I was notified an upgrade (from Nov. 25) is available, still 5.0.1 but a newer version. Is there any point to upgrading yet? It takes a long time.
The change is in the build, which went from .18 to .38. Not a significant difference, but you should be able to find the change log on blackberry.com/software (I had it saved but can't find it, sorry.)