You are right, I meant continuations, sorry.
HN is using URLs for passing those hash keys (if they are hash keys of course), but they need to store them on the server as well. So URLs themselves don't solve the problem in this case.
It's actually that HN doesn't use databases, where they could keep session-specific data much longer.
If you look at the URLs for voting on comments, they aren't using continuations. I don't see any reason why comments should.
The reply link I used to add this comment, had timed out, which seems wrong.
Have continuations just moved the the complexity -- making some aspects more elegant, while resulting in undesired side effects like expiring reply links?
I find that often happens when I'm designing abstractions - complexity moves somewhere else, and I later get unexpectedly bitten.
Surely they could come up with some way to store trivial state in the URL's GET params, without making life too miserable for the programmers.
Nearly 10 years ago, Nielsen wrote about "URL as UI" <http://www.useit.com/alertbox/990321.html>. URLs that break after an indeterminate time are not good UI.