Following on from previous posts on XenServer 5 issues, I've dug a little deeper...
Looking at my servers via the command line (xsconsole) I noticed some of them were saying their NICs were disconnected and the Make/Model was unknown, as before except XenConsole was now showing the opposite, i.e. everything was ok?!?
I decided to put the servers into Maintenance Mode and Reboot them, which resulted in the servers then becoming uncontactable!!!! 
So I had to visit the Console to see what had happened only to find the NICs had disappeared, never to be seen again! The only way I could think to fix it (quickly) was to reinstall Xen, which pleasingly took all of 10 minutes.
There are other issues though, like the fact that intermittantly the Netmask and Gateway settings simply disappear, as do the ntp settings except they are sometimes replaced with other settings.
I'm pretty sure this is all to do with driver incompatability, but if you look on the Citrix forums you'll see it's not limited to any particular Make/Model of server or even NIC. Also these issues didn't seem to happen on 4.1, which points to a wider problem introduced in 5.0, so all we can do is pray for a Hotfix of 5.1!
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