I started to put together a timeline of James Comey's tenure at the DoJ only to find out that emptywheel had already done one, so I'll just cop hers.  But listening to Comey's testimony yesterday, it was clear that the plan to fire the US attorneys was something they did not involve him in (other than to ask his opinion which they promptly ignored) and that they did not move on until after he was gone:

December 11, 2003: Comey appointed Deputy AG
December 30, 2003: Comey appoints  Patrick Fitzgerald Special Counsel on the Plame Affair
February 6, 2004: Comey reiterates  Fitzgerald's authority to investigate things–like perjury–that would become relevant in the Plame investigation
March 2004: Comey refuses to reauthorize  the NSA domestic spying program
Early 2004:  David Ayres (Ashcroft's Chief of Staff) asks  Comey for a list of weak USAs. Kevin Ryan is the only now-fired USA on the list.
February 17, 2005: Sampson recommends firing just some, not all, of the USAs
2005, unknown date: Comey provides  Sampson his list of weak USAs. Kevin Ryan remains on his list, but none of the other fired USAs are.
March 2, 2005: Sampson sends Harriet Miers a list grading all USAs. Patrick Fitzgerald is graded mediocre. Sampson revises this list after an AG Advisory Committee meeting; he adds two to the firing list, and adds one to the loyal Bushies
August 8, 2005: Boston Globe reports  on Black firing, in conjunction with Abramoff scandal 
August 15, 2005: Comey's Farewell Address  (If you haven't read this, do so now. Really. Otherwise Looseheadprop will be cross with you.) 
November 9, 2005: Moschella writes Brent Tolman, then Specter's aide, on inserting AG reauthorization into PATRIOT Act
November 11, 2005: Discussion of reasons to justify AG reauthorization, Moschella forwards to Tolman
December 14, 2005: PATRIOT Act reauthorization comes out of conference
January 9,2006: Sampson provides recommendations  on USA firings, listing 7 names, including Ryan
February 7, 2006: Federal Register publishes  Gonzales authorization of Sampson/Goodling personnel decisions
February 24, 2006: OLC tells Gonzales his order is unconstitutional
March 1, 2006: Gonzales signs an order  putting Sampson and Delilah Goodling in charge of hiring and firing our justice system
March 9, 2006: Bush signs PATRIOT Act reauthorization

It appears the plan was already in the works before Ashcroft left, which indicates that its execution was Abu's responsibility but that its genesis was elsewhere — and it's certainly not outside the realm of possibility that it came from the White House.  Did they think Comey would have balked at the plan?

Remember, Bush disliked Comey, who dogged him pretty much from the day he took office:

Miffed that Comey, a straitlaced, by-the-book former U.S. attorney from New York, was not a “team player” on this and other issues, President George W. Bush dubbed him with a derisive nickname, “Cuomo,” after Mario Cuomo, the New York governor who vacillated over running for president in the 1980s. 

They must not have been paying attention when Comey testified during his confirmation hearings:

I don't care about politics. I don't care about expediency. I care about doing the right thing. 

I guess they're just so used to churning bs they couldn't imagine someone might actually say something like that under oath and really mean it.