As Chris mentioned, here’s the response I got from Nike at the beginning of March when I asked them about when the challenges were coming back:
We will provide an update on the status of Nike+ Challenges soon. That’s actually all of the information that I have at this time, and I apologize I can’t give you a firm answer one way or another. Right now, we’re taking feedback that will be considered once a decision is made.
Your guess is as good as mine! Lets hope they get it back up and running (heh!). From the tone of the message it sounds like they’re thinking about getting rid of it entirely which would be a shame.
A few of us are still tracking our runs individually on Nike+ which can be motivating although maybe not as awesome as the group challenges. Feel free to add me! I’m always looking out for more virtual running partners.