There's no doubting that this will be extremely damaging for the Government. Whether or not Hain eventually clears his name, the public perception will surely be that corruption is endemic throughout the Labour Party - especially given that his was not the only name linked with malpractice. How many other names will be tainted by this affair, even if only in the public eye?
However, the nature of Hain's departure is potentially even more harmful than any spreading rot. The Spectator argued weeks ago that Hain should be sacked. Sacked, that is: shown the door rather being allowed to find it himself. Hain's resignation will lead to questions about why Brown didn't act sooner. Such questions will only amplify should Hain actually be convicted of any criminal wrongdoing.
Only 24 days into 2008, Brown's New Year relaunch is in tatters.
UPDATE: Hain's resignation statment can be read here.
UPDATE 2: Cameron's already questioning whether Brown should have allowed Hain to remain a Cabinet Minister for so long.