UPDATE: 9/18/12 - What ultimately happened to my preorder.
UPDATE: 9/12/12 - Amazon.com now accepting preorders.
UPDATED: 9/4/12 - SEE BELOW.
I sold my D3 and preordered a Nikon D600.
I am very sure that the D600 will be announced on or before Photokina, and I think there's a good chance it will be out by Christmas. I'm even more certain that the specs will be just right for my needs: a more reasonable pixel count than the 36mp of the D800, more portable than the D3 or D4 (I found the D3 too big and heavy). In terms of image quality, I believe it will have noise that is as good as if not better than the D3 and D700 but at a higher resolution (16 or 24mp) and therefore more easily cleaned up with noise reduction. Who knows, it may even have a pixel binning mode, which would be awesome (because low noise is more important to me than extremely high resolution). Plus it will have some of the new features from the D800 and D4. On top of it all, everyone is speculating that the price will be competitive. All of this adds up to one thing: the Nikon D600 is going to be a super hot seller. It will be harder to come by than the D800. Which means that unless you are part of NPS (Nikon Professional Services) or you pay a ridiculous premium on ebay or you preorder it, you will likely not get one for several months.
The problem of course is how to preorder it. If you call online stores like Amazon, Adorama and B&H, they'll tell you that you can't preorder it yet, because there's no item number for it (after all, it hasn't even been announced and is nothing more than a rumor right now). Instead, visit your friendly neighborhood camera store. Their policies are not as rigid as those of online stores. Ask them if you can preorder it or be on some sort of interest list. Perhaps they may even already have such a list. If not, then offer to put down a deposit on one. They might start a list.
If you live in Los Angeles, call the Bel-Air Camera superstore. I like them because the staff are very knowledgeable, they stock a lot of equipment, and their pricing is usually very reasonable. I figure since they're one of the largest remaining brick and mortar stores in LA, there's a good chance that they will have a decent allocation of D600's. I spoke to Michael and Rica. Both of them were very helpful and created a preorder list for me. If you give them a call, tell them you were referred by Mic Ty.
After that, you can join me for the vigil. :)
A word of caution: it goes without saying you should pick a reputable camera store. And obviously, nothing here is guaranteed.
In case anyone thinks I made this up, here is a screen cap of the charge on my credit card for the $300 deposit:
7/23/12 update: called Bel-Air Camera and spoke to Michael today. He said usually, Nikon will send their store a list of cameras that are available for them to order. So far, they haven't received any such list from Nikon yet. Meanwhile there is now a group of preorders. So I'm not the only one who has been able to preorder from them.
8/15/12 update: spoke to Michael again. No news yet re the D600. FWIW he also believes the D600 will ship in time for Christmas.
9/4/12 update. Called Bel-Air. They confirmed the news about a Nikon launch on September 13 although of course there's no official word on what the new camera will be. They confirmed that they will be honoring preorders for anyone who has placed a deposit. Once they receive the actual product, they will be calling people with preorders in the order that they were made.
Nikon D600 photos leaked; what we can deduce from the photos
Nikon D600 predictions
Nikon D600 order update