When a desktop shows up as “Agent Unreachable” or some other problem state in the View Administrator console, the first thing to try is to simply remove the VM from its View pool and re-add it. You might need to set entitlements again. It might take a couple minutes for View to register the desktop as available after you do this.
Use this solution any time you fix a desktop this way in View unless you know of a more specific solution that applies. Use this solution even if you have no idea why it worked.
One way that desktops can get into this state is due to new security settings in the View 5.1. These are applied to the desktop by the View server. If a desktop shows up in View during the wrong part of the mini-setup process, it may show up as Agent Unreachable forever, unless you remove and read it to the pool.
NP machines work a little differently. The most recent Unattend Builder automatically includes scripts in the Windows 7 SetupComplete.cmd that should keep NP desktops from going to unreachable on every reboot (_do these apply to persistent as well? please update this solution appropriately if you know!!!_). This applies to all NP machines in View 5.1, and also to XP in View 5.0 (see UNI-8852 for more information about XP and View).
If all else fails, you can try using the View 5.0 agent with the View 5.1 server.