Considering Google themselves promoted it with the Galaxy Nexus, I wouldn't hold your breath for a new Nexus phone anytime real soon. If anyone else watched the Google I/O keynote, you'll remember that they released the code, SDK, and PDK (platform aka hardware development kit) today, and the attendees' free GNex's will be getting updates this evening, despite Jellybean not officially launching until mid-July. And that the top 5 Android manufacturers (which of course includes Samsung) have had access to early versions of the code, in order to shape its development and have early access to begin optimizing it to their devices.
Considering these changes, and that Jellybean appears to be a very similar framework to ICS (though honestly including far more features than I expected), I would expect we won't have to wait nearly as long. Granted, Samsung will want to push their customized additions to ICS for a few months before overwhelming everyone with the new improvements in Jellybean (considering the vast majority of SGS3 buyers will just be getting their first taste of ICS). In fact, it wouldn't make sense for them to roll it out immediately, customers simply wouldn't be able to understand all the new features. But come late-summer/early-fall, when some of the features are a bit stale, and the new crop of phones, including the iPhone5, are on the horizon, a Jellybean update would be just the thing to reinvigorate SGS3 sales, and I think it would be feasible to have one ready in a few more months.
But yes, I would expect the AOKP and CM teams to bring us some Jellybean goodness sooner rather than later.