According to an AP story, it was postponed because of the Egyptian government's unwillingness to backdown over Ayman Nor's detention. US Sec of State Condi Rice more or less hinted she was considering not coming to Cairo for the conference if Nor remained in detention. He is currently being held for for forty-five days while the government insists criminal charges over forging al-Ghad party application papers are investigated.
While the US seems to be sticking to its guns so as not to appear out of step with its 'freedom' rhetoric, delaying the conference may actually indicate that some US-Egyptian agreement has been reached despite the difference of opinion.
The Egyptian government has too much invested domestically to call off the case. Also, election season is fast approaching and Nor's arrest will hurt al-Ghad's showing in those parliamantary elections. Meanwhile, the Bush admin has been taken a beating in the US press over its dealings with Egypt (via the Nor arrest). So perhaps this was the most amicable solution between the countries.
I don't suspect this will lead to a larger rift. Mubarak's annual trip to visit the US president is scheduled for April. If Bush keeps the appointment and welcomes Mubarak then the conclusion that can be drawn is that Nor will likely spend a fair amount of time infront of the courts in the near-future, Egyptian-Israeli-US cooperation will continue, and US proposals for democratization will take a backseat to other regional developments.
Either way, this latest twist will have little effect on the continuing oppositon and protests by the Kafaya movement.