My guess is that a dev release with developer tools, compiler, etc. will do two things.
First, it will let developers port 3.x/3.9 stuff so there are some apps ready for it and not just legacy versions running under the emulation layer. Of course, new code may be in the works too.
Second, a good way to find some of the subtler bugs is to have people writing for the new OS and see what snags they hit.
I bet they'll let the dev release run with the registered developers for a couple months and then release the consumer distro.
Just my guess....