Having maintained several Hyper-V R2 and virtual 2008R2 servers, I obtained the service pack and have learned the following from upgrading them...
* If you need to have RSAT installed on a Windows 7 workstation, do it before upgrading it to SP1: the installer should be updated soon; otherwise, the original installer will refuse to load on an SP1 workstation.
* You may need to shut down busy virtual machines before rebooting a Hyper-V server during upgrade: VMs that don't cleanly shutdown on reboot can hang the reboot for all the other VMs.
* You'll need to "Insert the Integration Services Disc" in the Hyper-V admin console for VMs older than 2008R2: this is so they can properly support the new features and hotfixes in the service pack.
* If you want to use Dynamic Memory, check your setting in Hyper-V admin twice: the max memory setting likes to stick on 64GB instead of whatever you set max to be.
* Some security updates came out after SP1 went gold: be prepared to reboot a 2nd time; especially on Hyper-V R2.
See also this blog article from Microsoft on using SP1 features.