I take a slightly different approach: I netboot diskless clients with OpenELEC[0] and share my media library via a read only NFS share. It works great, since its hard to find any device (thin or not) that doesn't have enough horsepower to decode media locally.

This can be combined with a UPnP approach if there are also non-dedicated devices that need to access media.

[0] - openelec.tv

That will not really replace what I use plex for.

I am running the plex client on dumb devices like tv's. That gives me a nice UI, sync of watched locations and automatic transcoding of content that the client cannot play natively.