In an approximately-simultaneous announcement, the Independent Election Commission of Afghanistan declared that no candidate in the election attained 50% of the real ballots cast, and Karzai rather generously agreed to the run-off.
The agreement is an about-face from his previous stance, which implied that he would dispute results that did not provide him an outright win. This, of course, would have been disastrous. Karzai may have, in fact, just facilitated the only route possible towards long-term governmental legitimacy in Afghanistan (though by no means, of course, did he guarantee it).
A number of obvious reasons are likely behind the decision:
1) Incredible international pressure, from NATO and the UN.
2) Fear of backlash, from Abdullah supporters and anyone else disaffected by fraud.
3) Hopes for increased legitimacy and popularity, by showing generosity and magnanimity.
Apparently, the run-off is scheduled for November 7th--only a bit over two weeks away. This will be necessary to make sure the elections are completed before Afghanistan's harsh winter hits, but will make logistics for ISAF, the UN, and Afghanistan's government incredibly complex.
More importantly: will Abdullah gain enough votes to make this a serious challenge? Karzai winning in the second round (with his cooperation) is certainly better than a "stolen victory" scenario, but it's unclear whether that can be avoided in the second round. Certainly, those working for Karzai have shown a ruthless desire to win (far beyond their desire to promote democracy). Will the scandal over the past weeks deter them from trying again?
It's unclear whether there is much hope to hold on to for this election. But if it should be "free and fair" and, most importantly, convincing, then the government may gain a new "boost" in confidence necessary to help the US "surge" in Afghanistan have a lasting effect.