MP4's work on the DMA 2100 extender using MyMovies, OML, Video Browser etc. The only thing I have noticed is a delay in playback, but theis may be due to Transcode 360. In my case, I started using DVR-MS video formats as playback is instant and FF/RW also work. For MP4's, I was using Nero Recode with 5.1 audio.
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Asus P5E-VM HDMI (onboard audio/video), Core 2 Duo E8200, 4 GB RAM, Hauppauge HVR-1800 (x2), LG-GGC H20L, DMA-2100 (x4), Yamaha RX-N600, InFocus IN76, D-LINK DIR 655, Vista Ultimate