It all depends
The Samsung ARM11 chip maybe used in iPhone and the Texas ARM Cortex and many others usually have built in TV and LCD controllers with on chip MPEG and AES engines too.
Some 266MHz ARMs SoC can do SD MPEG4 with no CPU overhead at all.
This could be rubbish or quite good depending on which SoC it uses.
Also if Linux can be loaded (almost certainly via JTAG, but that's not trivial).
My friend has Debian running on an ARM that came with Windows CE.