This means that any work that the Dev Team has done as far as progress into unlocking the 3G could very well be null and void if you run this.
Update:The newest PwnageTool (2.2.1) has been released and has been proven to remove the baseband update while preserving everything else. This is the only safe route possible right now... sorry, Windows users!
GOLDEN RULE: If you have a 3G iPhone and want potential soft unlock in the near future do NOT use QuickPwn, and do not use the official ipsw or the iTunes update process without using PwnageTool.
Download links for PwnageTool can be found via the Dev Team's website.
Source: iPhone Dev Team