VMware has just released an vCenter Server 5.1.0a update to address some issues found in GA release. I personally have had no issues with GA build in our small lab environment but there are people facing some trouble with new SSO service, and for that there are quite few items in the new vCenter Server 5.1.0a Release Notes.
VMware also released a new build version 838463 of ESXi 5.1 installation image to address ESXi 5.0 –> 5.1 upgrade issues on hosts with EMC PowerPath/VE 5.7 installed. New ESXi 5.1 build also addresses some CBRC issues with VMware View, but for full vSphere 5.1 compatibility you still need to wait for a new VMware View release. See KB2034549 for more information about patch.
You can find updated ESXi 5.1 installation image just below GA build on the “Download VMware ESXi 5.1 Installable” page at my.vmware.com.