I think people are missing the point. I don't see anything that indicates that Plex will do anything but SERVE content to the LG devices - not run on it. Something that other TVs (Samsungs, for example) can already do via UPnP/DLNA.
I have a new Mac Mini and a new i7 Windows 7 machine that I put together for the purposes of being a HTPC. Only XBMC on Windows/Linux is capable of delivering an acceptable experience on large screens. The media center options running under Mac OS X (Boxee, Plex, XBMC) are incapable of switching the refresh to match the source material. Judder Judder Judder.
I've settled on Boxee on the Mac mini for delivering local and network (TV) content to small screens, and XBMC on Windows for large (projector) screen presentation.