Let me first say that I'm a bit skeptical about the long-term prospects of the Darfur truce. I'm mostly not clear that the Justice and Equality Movement (JEM) and other groups in Darfur will be able to reach a sufficient common understanding that they can successfully negotiate with Khartoum as a united front.
The lesson of the day is that Realpolitik still trumps idealism in the international sphere. 7 years of grandstanding, diplomatic pressure, protests, and even International Criminal Court (ICC) action (which was largely ignored by Sudan and its neighbors) had done precious little to change the state of the war. Then why the sudden change? As far as I can understand, there are 2 key issues:
1) The Chadians are backing off. Khartoum regulars have seized sufficient ground in Darfur that Chadian action in the area is becoming more futile (and ultimately detrimental). Because of this, Chad declared an end to its proxy war with Sudan in the area, accepting a minor political defeat and retrenching in its own country. Without Chadian support, Darfurian rebels have a newly-diminished bargaining position. Sensing a potentially closing window of opportunity, Khartoum hopes to enter negotiations with the Darfurian rebels at their weakest, in order to end the war with the best-possible negotiated settlement (maximizing Khartoum's power in the area and over the country as a whole in the long-term). Such a favorably-settled peace will also boost al-Bashir's popularity, helping him secure victory in the next election.
2) Khartoum's priorities have shifted to Southern Sudan. Two big issues are coming up in Southern Sudan--the 2011 referendum on independence for Southern Sudan, and the decision on Abyei. If the referendum goes through, Southern Sudan would become an independent state (and given the current political organization proposed in the referendum, it would lead to massive political tension and possibly war). Khartoum wants its military forces to move south to be able to prepare for and deal with the fallout of an independence movement which has a great deal of popularity in the south. Furthermore, the region of Abyei is currently in dispute over where the north/south border lies. In particular, the dispute is over the location of the border with respect to a massive oil well that would either become northern or southern given its resolution (even if Southern Sudan remains part of Sudan, Khartoum ends up with more of the oil wealth than it would otherwise if the Abyei oil fields are located in the north). For Khartoum, a great deal of resource wealth is at stake in the south, where Darfur's threat is greatly diminished by Khartoum's relative control over the region.
Khartoum is ultimately moving on from the Darfur war because it largely won its political objectives and now has higher priorities in its path to political and economic consolidation. The takeaway here is that realpolitik still dominates action in the geopolitical sphere, despite the best intentions of the most powerful nations of the world. It's an important lesson to take away as we (the liberal West) continue to try to craft the world in our image.