VMware released vCenter Server 5.1 Update 1b and the vCenter Server Appliance Update 1b at August 1. This is an update with just bug fixes. No new features.
Read the Release notes here.
Download vCenter Server update 1b here.
It seems there is an issue with the ISO file when Update Manager is used to install the update. Read more about this in the comments of this post at Yellow-bricks.com