If you know which comments on a page are new, then you also know which ones are old. The easy answer is to just gray out the older posts (only remotely challenging because the gray background of the page is about the color I WOULD have suggested for old comment text) -- and just leave the new comments as they are.

That allows you to easily skim the page for new content, while still allowing you to see the new posts in context.

A Digg-style collapse would also probably work, but for some reason, I see grayed out text more in line with the ycombinator mantra.