[update April 7]
At April 21 VMware will unveil it’s new generation virtualisation platform. See http://up2v.wordpress.com/2009/04/07/vmware-vsphere-release-date/ for more info
Some expected VMware to release ESX 4.0 at VMworld Europe 2009 in Cannes. Although a lot of new products were announced, the new ESX was not amongst them. Paul Maritz said at Cannes the release will be ‘sooner than later’. Analysts expect a release sometime in May or June.
The Register calls Mays as a likely date for release according to the article on their site http://www.theregister.co.uk/2009/03/06/vmware_esx_v4/
Compellent comments on the news http://www.compellent.com/blog/post/VMware-ESX-40-Certification-Sometimes-you-can-believe-the-rumorse280a6.aspx?ref=HPBL_0306