Veeam has recently released this news on its forum:

Unfortunately, the hotfix release needs to be delayed until beginning of October for reasons unrelated to issues with Veeam code. The planned fixes were implemented and tested, but appeared not enough. During the final testing, we have found that full vSphere 5 support also requires switching to the new vStorage API version (VDDK 5.0) that only became available along with vSphere 5 release. This in turn requires significant coding effort (to implement new API version’s requirements), and major QC effort for full regressive testing across all supported configurations (as vStorage API is the cornerstone of our product, and this is the major release bringing many changes).

So, if you’re using Veeam and you want to upgrade to vSphere 5, please wait.