From what I have heard the originally targeted release date for VMware's vSphere 5 was August 5th. Now this has passed and it did not happen. There are now rumors ongoing that it will be released on August 22nd (see source)...
I don't know why it is being delayed. One possible reason is the change in licensing that was announced on August 3rd (see VMware's Power of Partnership Blog). With the revelation of vSphere 5 on July 12th VMware introduced a new licensing method based on vRAM (the amount of RAM allocated to running VMs) which lead to a storm of protest among customers and partners, especially because of the low amount of vRAM per physical CPU that was originally communicated. With the announcement above VMware has doubled this entitlement for most vSphere editions and they also capped the accountable vRAM for a single VM to 96GB (even if it has more RAM than that).
This will definitely help to speed up the adoption of vSphere 5 ... once it is released.
Update (2011-08-23): Okay, nothing again ... So it will probably happen on Friday (August 26th), just before VMworld 2011 (starting on Monday 29th).
Update (2011-08-25): It is out now, the official release date was August 24th. Customers with subscription go here to download. The free ESXi version is available here.