I unlocked mine(bought in April) using this nifty little site: http://rigol.avotronics.co.uk/mirrors/riglol/
Here's what you do:
1. Make sure your scope has the most recent firmware, when last I checked there was no update available, but just make sure that's still the case
2. On your scope, go to Utility -> System -> System Info. The little Pop-Up window tells you your Serial No, which you enter on the Keygen website
3. On the website, enter 'DSER' as your selected options, or pick the combo you fancy from the list below the entry fields(DSER gets you all available ones without the buggy 500uV range)
4. The 'Privatekey'-Field should fill itself, don't touch it
5. Click generate. Now you have a valid key to enter into the scope
6. Do the key-entering according to the normal Rigol User-manual: Utility -> System -> Options -> Setup ...
7. Be happy!
I guess this method has the great advantage of being pretty risk-free: Either the scope's firmware will accept the keys or it won't, I don't think you can brick anything that way.