I tried doing this on Gentoo linux.
I was able to get it compiling (i *highly* recommend using distcc + a cross compiling on a larger machine to save time), but it would segfault on launch. That was a few months ago.
I am going to revisit this, but in the meantime I am working on a simple python based player that uses iOS or android to control it (no gui, just a frontend server). You can take a look at https://github.com/dalehamel/pyplex
It works for basic playback right now. I am working on stabilizing it so that I at least have some kind of plex on the RPi. Once it is stable I will attempt the plex-linux port again.
It should be possible, but we will have to backport the XBMC patches that enable arm building properly. We will also have to force it to use OMXPlayer to play everything, as that is the only GPU accelerated player for RPi.