Matt Cutts announced that pagerank sculpting with the use of nofollows is becoming less effective, and would deteriorate further if they are misused. ...and they are.
This is not a surprise to most, but those that use nofollows and javascript onlclick links to protect paid links are in a scramble today.
The theory is that distribution of PR won't be isolated to just followed links. If you have 100 links on a page and 90 of them are followed, those followed 10 links will only distribute 1% each of the total page PR.... not 10% each.
Again, qualify all of this my remembering that this is relative to internal link development, not external. Google still applies weight to links on their own terms. Matt Cutts clarified in a later session that penalties would not be assessed for overusing nofollows. ...not today, anyway.
Nofollow does indeed stop googlebot from its investigative process, as does a robots.txt disallow statement. But, robots.txt disallows are not a substitute for nofollows, as they will leak PR.
From the same session, Canonical Link Elements are used as guidelines, and not hard fast rules -- much like other meta tags that were abused in the past.
