Meh, they can be annoying. but if top nav is static, I find it less annoying. It basically allows you to jump easily to sections independent of position, which is the same as multipage sites.
Assuming that you have keyboard focus in the page, but outside of any text fields or plugins (e.g. flash). Which might mean you need to find somewhere "safe" in the page to click that doesn't actually click something, like a button or ad.
I don't think this is what he means with infinite scrolling. Afaik, infinite scrolling is like Facebook, which loads new (older, actually) posts when you get near the bottom of the page.
Might be suitable for some contexts (like displaying a "feed" of individual items / search results / etc.), but then it's pivotal to provide users with an obvious way to hyperlink back to individual item. Think linking to one YouTube video (from your infinite "My subscriptions" list) or Facebook post (from your infinite feed). Most big sites seem to get this right, luckily.
I think Facebook do "nearly infinite scrolling" (to coin a phrase) as, IIRC, when you scroll for a while they stop and offer you a "load more content" button, possibly at year boundaries, maybe by number of stories shown, not sure.
