I think that for an amnesty to be viable, the Iraqi government either needs to be in such a commanding position that further opposition seems pointless (to the opposition) if just stopping is an alternative -or- the Iraqi government can draw into negotiations/discussions a sufficient number of representative opposition members (not necessarily the insurgents themselves, but local leaders who can be their proxies as political opposition rather than military).
Part of the issue is that because the Iraqi government making starts from a point that Sunnis and other minorities consider unfair (and the Kurds share the feeling though they are working from within the system, such as it is). Granted the difference is that between say “House of Representatives” fairness and “Senate” fairness - both attempt to give equal representation, but to different things. At any rate, having rejected the (UN-recommended, al-Sistani approved) system we set in motion, as long as the insurgents are not losing, they can hold out for a more than fair system. Juan Cole mused at his blog some time ago - hoping the Sunni’s see the misery the Lebanese Christians inflicted on their country trying to preserve their artificially inflated power within that country, then realizing that the Phalangists had largely succeeded in that effort.
Right now the Iraqi governement either needs a better stick or a better carrot than just amnesty.