I don't think that concession was binding.
I wouldn't be surprised if they fight.
If, and this is a big if, after all the counting and recounting if any the vote totals change, then the final count is what is certified by the states. The vote wasn't for president, but for presidential electors. Since this is a state certification, then the electors for the party that won a state are the ones that vote for the office in the Electoral Collage. You can bet that the democratic electors will vote for Kerry if they win a state...right now, I don't think any state's returns are "official". So, it is possible for thing to change, albeit unlikely.
If concession was binding, why waste time and money on continiuing to count? Absentees and provisionals could be projected unless it was close.
I think George Bush will be inaguated, but until everything is certified for him, don't think that the Democrats are through gaming every angle.