It’s a good thing CM9 reached the Release Candidate (almost final) stage in time before Jelly Bean got released, because it looks like the CyanogenMod team won’t be able to merge Jelly Bean that easily. The whole merger will consist of cherrypicking CM9 code when merging it with Jelly Bean to create CM10. Then they’ll work on fixing any possible conflicts and bugs.
The Jelly Bean code changes are not that big, which means it shouldn’t take another 6 months before we see a stable CM10 release; but they are still big enough that they need to be integrated manually. The CM team foresees the following issues with the merger, which will give them the most headaches, but they should all be solvable:
+ Trebuchet might be messy to rebase onto JB.
+ Theme Engine will be messy.
+ SystemUI stuff will need to be restyled.
+ Lockscreen stuff will need to be restyled.
This is why nightlies won’t be available for CM10 immediately, until they are happy with the state of the integration and of the CM10 ROM. The work on Jelly Bean should also not affect CM9, for which they plan a RC2 release, and then a stable release. But my guess is that will only take a few more weeks. It’s just in time before they finish the Jelly Bean merger into CM10, with all the latest fixes in CM9. They may start releasing nightlies soon after that!