iChat AV 2.1 Public Beta: "iChatAV21PanBeta Cannot be Installed on This Computer" Message

To successfully install iChat AV 2.1, quit any version of iChat that is already open before you begin the installation.

Quit the iChat AV 2.1 Public Beta installer, then quit iChat and install again if this message appears:

"iChatAV21PanBeta Cannot be Installed on This Computer. Please quit iChat before running this installer."
Note: AppleCare does not offer phone support for iChat AV 2.1 Public Beta.