I finally updated our productive environment to XenServer 6.1. The upgrade went without a hitch, but there are still many issues with Windows VMs and the XenTools. In the end completely removing the XenTools and reinstalling them did the trick.
Anyway, this post is about Dell OpenManage and SNMP. I've found a spot-on post in the Citrix forums here - thanks Josh Phillips. Read on for the details.