I believe the rest of the GOP candidates have. On the DEM side, only Obama and Clinton have opted out of matching funds and the attendant spending restrictions.
From a strictly pragmatic point of view, Democrats likely cannot risk running a candidate in the general election who has accepted matching funds IF that person is running against a GOP candidate who has not.
Thus if an Edwards or Biden begins marching toward the nomination and will be up against either a McCain or Romney, then there will be a very uneven playing field for about 5-6 months with the Dem candidate possibly being out spent about 10-1 over a period of many months.
Markos (of the Daily Kos) supports Edwards and will be voting for him in NH but has addressed this with some concern. The matching funds restrictions are crippling if running against an "opted out" GOP candidate. For those who are serious about regaining the Presidency, this should be required reading:
http://www.dailykos.com/storyonly/2007/10/2/125557/813If not for this, I believe John Edwards would be a strong and desirable GE candidate against a Huckabee, for example, who would be running with the same restrictions. In fact that would be great... taking about $300 million out of the Presidential campaign on each side and making those potential donations available for House and Senate races. But this worries me, truly.
(I have no hidden agenda and am not an Obama or Clinton "operative" so please no flames)