The Tivo Series3 is based around the Broadcom BCM7038 CPU and MPEG-2 decoder, but it also has a BCM7411 that is not yet used. The BCM7411 is the same MPEG-2/MPEG-4/VC-1 decoder used by the Samsung BD-P1000 and BD-P1200 Blu-ray players. It is also the same chip in the DirecTV HR20 DVR. The TivoHD has the newer BCM7401 DVR CPU which integrates the BCM7038 and BCM7411 into a single chip. The BCM7401 is the same chip found in the new DirecTV HR21 DVR. The DirecTV HR20 and HR21 DVRs both load a driver with MPEG-4 support, but Tivo only loads a MPEG-2 driver. I suspect Tivo will add MPEG-4 decoding capability to the next major release (maybe 10.x ?) so they can support high-definition downloads from Amazon Unbox (and elsewhere) in MPEG-4 AVC and VC-1 format. Unfortunately for Tivo, the hardware in the Series2 is not -- and never will be -- MPEG-4 capable.