If he'd just said "Yep, it was me" and resigned, he likely wouldn't have faced any charges at all.
That is not true. In fact, Fitzgerald said quite the opposite in his 1hr 20min press conference.
- He stated that Valerie Plame was absolutely under-cover, hence her status was secret
(1). Outing her would most likely be a violation of the espionage act.
- He stated that her outing was a huge blow to national security, and negatively impacts the work of the C.I.A., as well as future recruitment, since the protection of their status appears to be political fodder.
- He also stated that Libby's obstruction was like "throwing sand in the eyes of the umpire", and thus he is currently unable to successfully charge the alleged suspects on the more weighty crimes involved.
Either way, Libby lied for a reason...and it definitely wasn't because he got a hummer in the Oval Office. He's either covering for himself, or much bigger fish (Cheney). If he would have said "Yep, it was me"...he'd be facing way more serious charges, that he's so far been charged with perjury/obstruction shows he thought it serious enough that he'd blatantly lie to a Grand Jury.
Personally, I think this is FAR from over. Watergate started much in the same way.
(1) Count 1, item 1f: At all relevant times from January 1, 2002 through July 2003, Valerie Wilson was employed by the CIA, and her employment status was classified. Prior to July 14, 2003, Valerie Wilson's affiliation with the CIA was not common knowledge outside the intelligence community.mrC
EDIT: Added relevant item from Indictment.