under are the terms you can be fired under.

If and this is a major if, IF the standard holds true, then since there was no Senate confirmation, there can be no impeachment. Clinton fired a couple of USAs, at least one 'for cause' (physical violence), and I think that is the way Griffen would have to be treated.  One would have to find cause in order to get him fired. Example: get him convicted of a felony (such as caging).  I don't know what the time limitation on caging charges are, maybe only the 2006 election would apply. It should be noted that the entire Republican Party is under a cease and desist order dating from 1986 regarding caging, so that may be an avenue to explore.

Take this post with a major grain of salt because the regular hiring practices for this USA were voided and the rules seem be being made up as the Bush folk go along. In light of his words with Pryor, Griffen does not seem like a man likely to resign, unless a major kleig light is shown upon his work.  

Remember how Griffen said he was not going to put his name before the Senate, and most people backed off, thinking that he meant that he meant that he was not putting his name forward period? He DID go forward with out nomination or confirmation, and he is still there AFTER the 120 day interim period that Abu talked about. Will Congress will have to query the DoJ Office of Profesional Management for a ruling on how to get rid of Griffen?