I honestly don't believe that the new Nexus phone will be out until Key Lime Pie either. I'd love to see a new Nexus phone this fall, but I doubt it. Google, please prove me wrong!
Nexus devices usually come out as the flagship for a new OS version. We have the 4.1 flagship with the Nexus 7, and the Galaxy Nexus seconds as a solid standard-bearer for Jellybean as well.
The fragmentation issue still exists, but it appears to be shrinking. First of all, the phone/tablet fragmentation has now been eliminated. Honeycomb is gone, and tablets now run the same stream of OS as Android smartphones. That's a big step in the right direction right there.
On phones, the Galaxy S 2 took about 7 months to get the Ice Cream Sandwich update (from November 2011 until ~June 2012). I had a GS2 (international version), the wait was excruciating.
On tablets, the ASUS Transformer TF101 took about 4 months (Android 4.0 came in March of 2012).
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Jellybean just came out. We've got announcements from Samsung that it should be coming soon to the Galaxy S 3 and ASUS saying that their tablets (at least the TF201 onward) should be getting Jellybean relatively soon.
Android still has a huge lag compared to Apple in their release of new OS versions trickling down to older devices (and in the case of the Galaxy S 3 and One X - not so old devices). When Apple releases a new OS, you can download it right away for every device that they will make it compatible for. However, Google seems to be moving in the right direction. The Galaxy Nexus got Jellybean around the same time that the Nexus 7 came out.
Other manufacturers will hopefully follow suit quickly, otherwise I predict Google's own Nexus brand devices, which are essentially assured to get subsequent Android releases, should gain in popularity, which will force manufacturers to adopt a more agressive OS upgrade policy or lose sales.