I knew this was the case, but count me in with the group that wishes Sony and just let the rights lapse.
The same contract stipulation exists with Fox having the rights to the Fantastic Four. I know that they have to have another movie by a certain date or the rights revert back to Marvel on that franchise as well, which is why Fox has a FF reboot planned for 2013. (which I suppose allows them to retain the rights)
I wish it would happen, too. When you look at how a guy with amazing comic world credentials like Kevin Feige really helped steer Iron Man (and subsequent Avengers related projects) in such a positive direction for Marvel, you can see that there is the potential to do right by these movies. The first FF movie was ok in it's time, but Iron Man just made it look like dog**** in retrospect. I would love to see the way Kevin Feige could steer FF or Spiderman. And the potential to have a completely contiguous Marvel film universe is really tempting.
But when over $2 billion in past earnings was on the table for Sony with Spiderman? I can see why they opted to hold onto the rights for Spiderman. And Fox will likely hang onto X-Men, too. They've made a lot of money with that franchise, and thankfully "First Class" wound up being a pretty good movie.