I upgraded our lab vCenter Server to 5.1.0a version and noticed that this upgrade broke vCenter Server integration in vCenter Orchestrator.
During vCenter Server upgrade Orchestrator simply removed vCenter Server plugin silently, it took me a while to figure out why all my workflows suddenly stopped working. Once I figured out what was going on I started looking for a new vCenter Server plugin, first I went to VMware Solutions Exchange and found out that vCenter Server plugin listed in there is an old version (5.0.1), if you install this plugin in Orchestrator shipping with vCenter Server 5.1.0a you get "Installation corrupted (bad plug-in version)" message.
Eventually I found a vCenter Server plugin version 5.0.2 at Download VMware View 5.1 in my.vmware.com but this plugin is also reported as bad plug-in version with Orchestrator 5.1.0a. Btw, this vCenter Server plugin version was really hard to find as it is not listed in "vCenter Orchestrator Plug-ins" menu on VMware vSphere 5.1 downloads but it is listed in VMware View 5.1 downloads!
So, situation is that I cannot find any working vCenter Server plug-in version for Orchestrator 5.1.0a at the moment. Hopefully updated plugin will be released soon.