Provisioning Services 5.1 has been out for awhile now and it amazes me how many people are still running 4.5 or 5.0. Why haven't they upgraded yet? Do they know about the new features?
User Assigned vDisks - You can now assign vDisks based on Active Directory or Windows groups.
Auditing - You can now record and review administrative changes to the farm. Find out who did what and when to troubleshoot the Provisioning Services farm. It's kind of like Configuration Logging in your XenApp farm. This isn't enabled by default. You have to enable it on your farm.
Standard Mode NIC Teaming - Teaming Intel and Broadcom NICS are now supported.
Read-Only vDisk Storage - You can now have Provisioning Servers share the same LUN from a SAN without the need of shared file system.
Multiple vDisk Partitions - Provisioning Services 5.1 has removed the image builder utility and replaced it with XenConvert. You can now create a multiple partition vDisk very easily using XenConvert.
Offline Database Support - Target devices now have database cache. If the database server goes down, they will continue to run. This is kind of like local host cache in XenApp. This isn't enabled by default. You have to enable it on your farm.
Offline vDisk Maintenance - You no longer have to reverse image back to a regular disk to update your network drivers. You can now use Hyper-V or Virtual server to update your vDisks offline. One thing to note about using Virtual Server to do an offline update is that it doesn't support multi-core hals. Virtual Server only supports single core hals and can't boot a multi-core hal VHD. For the Hyper-V offline process see the Citrix blog - New Way to Upgrade with PVS 5.1 and HyperV. Another cool feature about using Hyper-V for offline maintenace is if you install Hyper-V's Integration Services you can boot the VHD from both virtual and physical machines. How cool is that?
With all of these new features why haven't you upgraded yet? It's definitely worth a look!