It may seem like odd timing, but it makes a perverse sort of sense. Come Monday, the Department of Justice will be required to release documents they previously withheld from Congress during the Fast & Furious investigation. Holder may be trying to distance himself from whatever fallout might ensue, but he remains in contempt of Congress for that bit of obstruction.
While it looks like he'll escape any consequences from that (and it has for some time), one can only hope this will haunt his professional life. Two American law-enforcement officers and over 150 Mexicans died as a result of a botched operation that took place on his watch.
As it stands, we've got a midterm election coming up, and there's a possibility of the Senate changing hands in January. If that's the case (and I'm not sold on it), the President will have a difficult time appointing a favorable successor. Expect a nominee before that.
I imagine Solicitor General Verrilli's name will be in the running. He's the safe choice. Deval Patrick's name was briefly floated, but his political baggage is more disastrous than Holder's.
Kamala Harris is also a possibility, which concerns me. She instituted California's ballistic fingerprinting system, supported Proposition H (a total ban on handguns in San Francisco), and signed on to an amicus curiae brief in the Heller case arguing against the individual right to keep and bear arms.
We're not going to get a good appointee, but Verrilli seems the least dangerous possibility.