I opened my hosts file (C:\Windows\System32\drivers\etc\hosts) and set mint.bitminter.com to resolve to one of the IP addresses for mine2.btcguild.com (107.21.113.86). The idle BitMinter client immediately asked for new login credentials.
I had signed up at BTC Guild as WhitePhantom, so I just had to change my worker name in the BitMinter client to "1" and the password to "123".
The BitMinter client started mining on BTC Guild, although I was getting about a 10% reject rate for whatever reason. Still better than being idle, though. I came back over to BitMinter after about 4 hours and am glad to be back.