The google indexing is absolutely true. I've tried to time it, usually it takes only about 45 minutes from the moment it gets posted to being indexed. You can see how long it took by searching in google for the hn submission and then check to see how long ago it was spidered, versus how long ago it was posted.
The google bot must re-visit hn very frequently, 10's of times per day for the 'news' and 'newest' page.
Good one. I don't know the exact implications of nofollow, but I think it might be that if google considers the destination to be legit that it will add the url to its crawl queue without giving it any 'pagerank juice'.
Only a googler could tell you for sure though.
I've seen google ferret out results that were never linked through the toolbar or some other mechanism outside of the regular crawl process. (in that one case the toolbar was my only possible explanation, or it was through analytics).
And I guess there's always google juice to consider - may be more relevant to pages about Hackers, but if it's being indexed that often than all the sites HN links to must benefit from association.
(I think this comment really shows my SEO naivety, but I hope you get the gist.)
I think the HN homepage pagerank is diluted too much to make much of a difference, first of all there are 30 other outgoing links, and besides that they change all the time.
Google 'juice' is most likely best collected by having links on authoritative pages that will keep the link visible on the same page for a long time, and that have relatively few outgoing links.
The google bot must re-visit hn very frequently, 10's of times per day for the 'news' and 'newest' page.