If the OS isn't released to developers until mid 2011, it's possible that Apple releases it at the same time - I wouldn't put it past Apple to do that to developers.
At the same time, if Apple delays it too much, it will allow Microsoft to get Windows 8 out of the door before Apple can get 10.7 out, potentially letting MS once again out-shine* Apple in their OS release.
*I simply mean that the work that MS did to Windows far more noticeable than any work that Apple did to Snow Leopard - This is not a comparison of OS's, but a comparison of which matured as compared to their respective prior release.
/Flamesuit
Haha, I almost fell off my chair. Although the majority of users still use MS, I personally think Win7 was really a "oh no people hate Vista" release. Yes, what MS did to Windows IS far more noticeable, but I don't think anyone will think OS X is lacking in comparison to it. If you are comparing the maturity of the OS, then Snow Leopard matured a lot more compare to what windows 6.1...err i mean 7 has to offer.
But you are right, the things that Apple refined for snow leopard is really under the hood, so is is not easily seen. If maturity of an OS is only dependent on the "eye candy factor" of what the UI has to offer.
For example, lets look at Expose vs the window Snap on 7. Typical OS X users opens up many applications all at once, with the most recently accessed window on top (focus). Expose (especially the way that I have hot corners set up) allows you to quickly see all your windows at a glance, and pick the one you want. I even have another hot corner set to show the desktop so I can quickly access files there. Everytime I show non OS X users this, they all say "OMG That is cool, can I get that on my (windows) computer?" (the answer is yes btw, but its nowhere as smooth)
Now, in traditional windows, you would Alt Tab the windows until you find the one you want, and then choose that window. Windows Vista (and now 6.1... er i mean 7...), the flipping windows (or carousel?), its just traditional Alt-Tabing in disguise. Windows 7's Snap feature is worse. ok, so you have 2-3 web browsers opened, maybe iTunes opened, Microsoft Word opened. What Snap is trying to do is make a program take a side of the screen and Snaps to it... after all the screen real estate is used up,each windows' size is smaller, the current focus still block others behind it, and there are still no efficient way to switch applications. (and i compare efficiency with expose... so its not a low bar at all)
Just by comparing expose vs Alt-Tab/Snap alone, you can see that Apple put a lot of thought into its OS (now i didn't say Expose was invented by apple).