VMware have already published a decent amount of information onto vmware.com around vSphere 4, in anticipation of May 21st when it will become possible to purchase licenses for the software for the first time.
The Upgrade Center for vSphere 4 contains resources around the new capabilities in the product, strategies for upgrading from VI3, the new license pricing and packaging, links to various migration videos, and more. Here's the link:
We still don't know exactly when the software will be available for download, but we do know that the VCP exam will be available shortly after, and that training will be available from the end of June.
I'm away on holiday for the next couple of weeks, hopefully by the time I'm back the GA date will have passed and I'll have a whole load of new information I can publish :-)