Can I use a Mid 2011 27 inch iMac (Thunderbolt) as a display for a PC?
No - currently, only computers with a ThunderBolt port (combined DisplayPort + PCIe data) will work for video in mirroring on an iMac. This means a small selection of other Macs that ship with thunderbolt currently work.

There could be adapters later that help, and perhaps a firmware update, but even a MacBook Air that you buy today with DisplayPort out won't work with the iMacs expecting thunderbolt in.