The reasons for the Gonzales departure may not be completely obvious.  Clearly, the main reason is that virtually the entire government (probably including Cheney), but probably not including George W. Bush, has lost confidence that Gonzales can wait out the next sixteen months without bring more discredit upon the administration.

Having said that, that Gonzales is fundamentally incompetent, the more important reasons are that Gonzales has become a lightning rod for impeachment.  Had he not resigned he surely would have been impeached, breaking the moronic notion of the Democratic Caucus that these criminals in the White House and DOJ (and elsewhere) are too petty to bother with when the objective of ruining the Republican Party is so tantalizingly near.  Pelosi surely would have heard from her constituency in San Francisco by now that the Rahm Emanuel-led Democratic Caucus is fundamentally wrong about impeachment.  Conyers would have been given the go-ahead and the Democrat's virginity on impeachment would evaporate.

Of course, Gonzales is still impeachable, but being out of office makes the call all the more difficult for the Democrats.  Frankly, I think they do not have the moral fiber to do it.

The importance of slowing or thwarting the Gonzales impeachment then is that decision to impeach Cheney and then Bush remains all the more difficult to make.  You can be very sure that the Democratic Caucus Old Bulls will hold to the idea of rubbing the Republicans' faces in all the mud and corruption they have created over the past six years rather than go through a constrained and formal process of impeachment, especially when the votes in the Senate are not assured.

If there is sufficient momentum to impeach Gonzales, even then the Old Bulls will probably have their way.  It must be remembered that when DeLay and Gingrich were in charge they treated the Democrats like excess baggage, ignored them in planning, insulted them daily, and the Old Bulls will never forget that.  It is matter of personal revenge for them, the Constitution be damned!

There is no telling what role Gonzales plays in the day to day business of the lame-duck Bush White House.  But, clearly, with Gonzo gone and Rove retired, Bush is more and more in the clutches of Darth Cheney.  I think the situation sets up nicely (for them) to propell the U.S. into armed conflict with Iran.  Such a move, which is easy to concoct and  perpetrate—consider a Gulf of Oman Resolution or Kurdish Freedom flap—will not require either Rove or Gonzales.  Bush and Cheney will retreat further into their bunkers and hand over a real debacle to the Democrats in January 2009.  They will do that, if they do not in the meantime declare martial law and stick around for another eight years!