If Sony are to launch first, wouldn't their chips have to be in production now?
No. Summer time, although I guess maybe earlier in the summer.
Doesn't the lack of leaks on their silicon production and developers not having final dev kits mean that there's no way they can reach market in Sept-Oct?
Not necessarily. Final kits on previous systems have arrived in some cases with just a couple of months to spare. I think final 360 kits cut it very close to launch (?)
Not that I'm also not skeptical. But these aren't dead giveaways.