The Samsung B550 is a fairly cheap set where this feature can be enabled but the B650 is much nicer if it's within budget. The higher end LG's (LH5000 and LH7000) also have this feature. I'm sure there's more sets that someone else can chip in with
Plasma wise the LG PQ or PS6000 have usb movie playback.
I watched a dvdrip avi movie from my usb drive on my LG42PQ6000 for the first time last night, and it was flawless, really impressed with it.
So much so, that i'll be creating a library to put onto an external 2.5" hdd with high quality avi's on.