Coming straight from the iPhone Dev Team’s blog.
Update warning!
Now for something a bit more serious, we’ve explained about the pros and cons of applying Apple’s updates in the past and we’ve warned against the immediate installation of these updates without knowing what they do to your device.
This is indeed true of the iPhone 3G’s upcoming 2.2 release. Installing ‘2.2’ straight away on the iPhone 3G using the iTunes auto-updater could affect your chances of any software unlock in the near future (should one be found and released), so when you see an update in iTunes await our instructions first!
Please don’t blindly install the update and then complain about it later
As stated above, there is a possibility of not being able to ever Jailbreak your phone again after applying the 2.2 firmware from Apple. If you remember the 2.1 firmware update which solved a lot of issues regarding battery life and application stability. There was only the warning that you would loose your Jailbreak if you updated. The warning wasn’t as specific as “Installing ‘2.2’ straight away on the iPhone 3G using the iTunes auto-updater could affect your chances of any software unlock in the near future”. It looks like the Jailbreaking game is getting a little more serious.
Source: iPhone Dev Team Blog article “Unmasking”