Not really a big surprise; China's clearly been putting on pressure, and North Korea really exists on China's sufference. The DPRK probably knows that they're in a better negotiation position than they were before, too- promises of a "hard line" by the U.S. and Japan will run right into the reality that North Korea has a lot more chips than they used to, chips that the Koreans will almost certainly used to gain concessions.
Still, I think at this point that there's only one concession that really means anything to Kim Jong-Il- the assurance of non-hostility and security by the United States. This whole mess started because (among other reasons) the United States has waffled back and forth between killing time until the DPRK falls and threatening to do the job itself. I'm sure that Kim is looking for some proof that his regime is safe; everything else, including the economic concessions, is secondary.
For its part, though, the U.S. may not be willing to offer this pledge. After all, if Kim cheats, the United States could see North Korea end up with nuclear-tipped ICBMs, and clearly they don't want that and will preempt it if they can. Yet it's that fondness for preemption that helped create this mess in the first place, and they can't simply set aside Kim's concerns.
It'll be interesting to see where this goes.