My own point of view:
The lead-time required by Amazon and other online stores means that WotC pretty much has to give about eight months' warning at this point; they won't be able to try a surprise release, and they won't want to stretch it out longer. I'm not sure how well the year of transition from 2E to 3E went for them, though.
Therefore, my guess would be that we'll see the announcement at one of the two Big Events--D&DE or GenCon--with the release following at the other one. Given what we know of the release schedule, I'd say that WotC will announce at GenCon 07, D&DE 08 or GenCon 08, with a release at D&DE 08, GenCon 08 or D&DE 09. They may hold off a little longer, but I think 3.5E is hitting the point of collapsing under its own weight, and they want to do some dramatic things (consolidated class and skill lists, removal of the 'ivory tower design' and 'hidden system mastery' elements, revamp of spellcasters) that will require a rebuild of the core.
I do find it interesting that for the first time in several years, they've announced the whole year's D&D product line in one fell swoop. For the past few years, they've been running on a rolling trimester model. Perhaps they want to give themselves some breathing room as they prepare for the launch?