Be careful though. Depending on what your version is, you probably want to build a custom IPSW with PwnageTool so you don't lose your unlock.
If you accidently upgrade you can still unlock, but not without issues. You can replace the stock 4.2.1 baseband with the iPad one, but it makes some pretty major and permanent changes (and breaks a few things like GPS).
With a little bit of planning and assuming you haven't already just upgraded to stock, you can avoid this and keep your unlockable (and perfectly working) baseband but upgrade to the latest firmware version.