I picked up a Sony KDL55EX645 today. After installation I took a USB thumb drive, formatted it FAT and loaded mkv, avi, jpeg and mp3 files from my Mac.
I attached the USB drive to a USB port on the EX645 then using the remote chose HOME>Media , selected the USB drive and then Photo, Music or Video depending on the file type I wanted to play, then the appropriate file. They all played perfectly. No need to convert.
Edit: There seems to be a file size limitation with FAT formatted drives. exFAT or FAT32 is a better option.
Also, mkv files play very nicely if I stream them to the TV through a PS3 using PS3medialserver. This option has the added benefit of being able to handle dts audio,