It seemed to appear too quickly, and I was suspicious that it is going to be a band-aid on a gushing wound... next to useless, 'cos it wasn't thought through. Adding the attribute rel="nofollow" to links in comments, will not prevent comment spam, it will merely change the spammers' behaviour.
This post on kuro5hin.org confirms my suspicions.

Spam 101

I'm going to take a bit of a risk here and go on record as saying that this will not solve comment spam at all; in fact, it'll probably make the problem worse. The reason for this is that there are currently two kinds of spammers:

* One kind, let's call them "Group A", spam weblogs and discussion forums for the PageRank bonuses.
* The other kind, "Group B", spam because they want their address or brand name to be seen by as many people as possible. They don't care about PageRank, they just care about the (fairly constant) percentage of people who will hit their site after seeing the address or name. This is the same motive and method we're all familiar with from USENET and email spam.

Obviously this can stop Group A in its tracks if widely implemented (and the Google announcement sports an impressive list of weblogging tools which are already on board, with more likely to follow), but it does nothing about Group B. What's more, the Group A spammers are unlikely to say "Aw, shucks" and give up; they're probably just going to become Group B spammers, because the marginal gain of visibility is better than nothing.