Simon Willison seems to spell it out the best:
Reading between the lines (which in this case isn't
particularly hard), this
and this (don't
forget to view source) suggest that Google are soon to announce that they won't
be calculating PageRank for links with a
attribute. Finally, an official way of fighting the economics of comment spam by
denying PageRank on user-submitted link content. Sam Ruby points
to Mark Pilgrim's prediction
that spammers won't care - they'll spam anyway, on the offchance that they hit
somewhere undefended. I'm optimistic - if the major weblog (and wiki) vendors
get behind this one it could help stem the tide.
This is cool - it lets content link sources control the page rank boost they give to the link target - perfect for comment spam, forum spam, etc. Let's hope that Mark Pilgrim is wrong - if this really does make comment spam ineffective, maybe it will at least slow down a bit.