What did he know, and when did he know it? Or, if you're feeling more pessimistic about the future of our current AG, Alberto "Fredo," "Geneva Conventions are quaint" Gonzales: When will he go, and how will he do it?
With Republican senators offering at best lukewarm support, Fredo's days may, indeed, be numbered. But the senators seem to have fixated on the need for Gonzales to clear up the record about his involvement in the firings. As others have pointed out, this is only part of the cover up. The question at the heart of the matter is why were the USAs fired in the first place? Even admitting that it was for 'political reasons,' as Trent Lott seems very willing to concede, does not really answer the question.
The real answer, I suspect, is a combination of the mundane and the Machiavellian. Look for the White House and DOJ, once they regain their senses in the face of actual or threatened subpoenas from the House, to emphasize the mundane half of the explanation: that the firings were just part of an effort to build the Republican bench of people eligible for higher political and/or judicial appointments. They fumbled this very badly, of course, in the way the firings were handled, and the lying to cover up their fumbling may still cost Gonzales his job even if the 'bench-building' explanation is accepted by the media and public.
The more sinister side of the firings involve efforts to derail investigations into Republican corruption, and to punish those who would not play along with the GOP's phony voter fraud charges. Most likely, Rove saw the chance to kill two or three birds with one stone (or eight stones, as the case may be), and the USA's were dismissed for a combination of mundane and sinister reasons. Alas for the White House, they seem to have grown to used to a rubber-stamp Congress and press, and figured they could slip this by without attracting attention. (I'm not saying we've yet regained a fully-functioning opposition party and fourth estate yet, but there have been improvements.) If they had just gone with the bench-building explanation in the first place, they might have succeeded, too.