I think the ARM core is more about networking related services, although it remains to be seen if the ARM does the video encoding or the AMD, seeing as both can come with the necessary hardware encoding support. I suspect the AMD though as it has hardware assist in there as standard so why add the complexity of finding a way to give the ARM access to the frame buffer, when the AMD already can do that.
A good point about that 15 minute buffer but seeing as there are PVRs using 2.5" HDDs I don't see why we should discount the HDD as being used for it. That said, I see no reason why they couldn't fit the entire background OS and 15 minutes of encoded footage into 1GB of RAM. Its one of the reasons the Wii U OS frustrates me so much as it should be lightening fast with all that RAM reserved for it.
Edited by Alex Atkin UK, 11 April 2013 - 03:24 PM.