I think this might have implications way beyond blog comment spam. Not sure what yet but here are some quick uninformed thoughts:

Obviously, links are a big part of Google, and other SE's results (PR, anchor text, indexing, etc.). By making this tag available, in effect, Google kind of gave the control of their SERPs back to the Internet. Big chunks of the net can now unlink to everything that is not a human reader. Once they know it's available, corporate America / big time websites might make it a policy to automatically use this tag for every external link. You know how they are with the net. Picture the meeting with their hunt and peck managers – "Absolutely, they are not paying us – no follow everybody that doesn't pay us." And why shouldn't they?

Wouldn’t that kind of break the SERPs though? – It would at least influence them. What if the Yahoo directory and DMOZ added these tags to all of their links – wow

Blog comments, I don't know – One thing that encourages some to comment is that they will get a link back to their site. Will the casual blogger bother commenting anymore? Why would they do a trackback? Sure it still connects the visitor that day but it would no longer theme those relevant pages together in the search engines (short term vs. long term benefits of a comment /trackback). That LiveJournal solution sounds sweet though.

One use for the non-blog site, if the tag counts for internal links, is spreading the PR internally throughout your site. Links to pages where you do not need PR can now be almost cloaked from the bots – so add the tag to all of your privacy policy, contact us, and store locations type of links so the PR transfers where you want the juice (like to your product pages). I think this is a great use of that tag – Just because my "enter your zip code" tool has a link from every page on my site, I don't need it to have a PR of 9 when my product pages have a measly PR of 8. Not that PR on its own is that important anymore but still. (That was only an example BTW – divide those PRs by 2).

Most external links on your site should probably not be changed. On topic external links help visitors and might boost SE rankings as well. Example: a page where I am selling blue widgets has a few prominent non-reciprocal links to related blue widget info sites (a blue widget authority site, a link to a Google and a Yahoo search for blue widgets, etc.) That, combined with some organic anchor text from an external site for blue widgets makes it clear what the page is about no matter what flavor algo the SE uses. After all, these pages are built for the visitor and not the SEs.

There is always some good and some bad that goes with change, I am still chewing on this one. – Off to go change some privacy policy / contact us links and add another layer to my tinfoil hat.

Very good points. I'm now pondering to add the "nofollow" to such internal pages which are linked from every page, but which aren't critical in terms of finding them from the outside. But one thing: if those non-critical pages gain a good PR, and they include backlinks to your main page, it will also give some PR back. So I'm not sure if the "link mojo" is really lost in terms of the whole site?