Once you've updated your Mac to the new release, you'll find the HP MediaSmart Server client can still detect the home server on the network, but is unable to remount the drive – leaving you to work through the same manual workarounds as other Windows Home Server v1 owners.
With the MediaSmart Server team at HP now disbanded to work on webOS devices, we'd be surprised to see a compatibility patch released by the company. Windows Home Server 2011 is due to support Mac backups courtesy of a (still unreleased) third party add-in. HP MediaSmart Server owners currently backing up their Macs will now be faced with the dilemna of holding fire on their OSX Lion updates later this month, or plumping for a WHS 2011 upgrade.
Thanks to Gatsby Assassin for the tip.