Given everything else they're doing with Apple TV, the least surprising explanation for the iOS/A4 thing is that it's just easier for Apple to build new products on that platform. It's probably not a major strategy "tell".
I agree with you, that was just the expand on the ‘competitively priced’ point. The use of iPhone components (both software and hardware) doesn’t seem very indicative of any big strategy change to me, either.
It looks like you get the iPhone (or iPod touch) guts [+] without screen, battery, cameras and probably a lot less persistent storage (I’m guessing anything between 512MB and 2GB) in a bigger box (most of that space is probably used by the PSU, though). That might just give you a margin when you sell it for $100, but probably not a big one.
I don’t think it will be a hit but I do think it will be more successful than the predecessor.
[+] Gruber seems to have a source that tells him that Apple TV uses iOS and the processor is a A4.