My very early experiences were positive but the wrinkles are now only being worked out.  My advice for using VMM 2008 is this: deploy it before you deploy Hyper-V.  Adding an existing Hyper-V cluster to VMM 2008 is not recommended by me.

The first thing I'm going to talk about it virtual networks.  I have an existing Hyper-V cluster.  I created 2 virtual networks (mapped to 2 NIC's) on each host.  Networking on very host was configured and named identically at every level by me.  The failover cluster validation report was a pass and VM's failed over and back while maintaining network connectivity.  Perfect!

Then I added VMM 2008 to the mix.  It sucked in the cluster and deployed it's agents.  The first problem (which I'm still trying to resolve) is a Win-RM access denied issue that appears after a host has been managed for several hours.  I've no idea why.  I've got a call open on this with MS so I hope to post a resolution at sometime soon.

The second problem is the one I'm going to talk about now.  After a while my highly available VM's started going red with a status of "unsupported cluster configuration".  I knew the cluster was OK because of my report and because of how the VM's moved OK.  I was getting an error on my VM's telling me my networking was at fault.  Anthony Crotty sent me up a link that described a scenario when this error occurs.  It wasn't identical but it did point out non-identical networking across the cluster hosts.  As far as I could see my networking was identical across all the hosts. 

There's a property box for a Hyper-V cluster in VMM 2008.  In there you'll find a networking tab to show networks that reside across all the hosts.  My 2 virtual networks weren't there.  Intriguing!  I added a test private network to host 1 and it was automatically built across the other hosts.  That's handy!

I deleted virtual network #2 and rebuilt it on host 1.  It was recreated on the other hosts and now it appeared in the cluster properties networking tab.

That leads me to this theory.  If you create virtual networks by hand (as you have to if you don't yet have VMM 2008) then there's a hidden configuration that's required by VMM managed clusters that is not created.  I'm thinking there's supposed to be a common ID in the hidden properties of the virtual network across the hosts.

So I recreated my 2 virtual networks, mapped the NIC's and configured trunking on host 1 and they were created across the hosts.  I did a quick test, luckily.  VMM 2007 may set up the virtual networks on the other hosts but it did not map the NIC or configure trunking.  I replayed those steps on the other hosts.  You have to be patient when doing this.  You may see the dialog disappear when you click OK but there's a job still running in the background to carry out the configuration.  Don't jump straight back into the dialog box expecting to see your new configuration.  Watch the properties of the host update and wait for the job to complete.

Hopefully I can post something a little later about WinRM.  I expect it'll be something stupid like the above.