A recent Google Groups thread draws attention towards Google's way of handling nofollow attributes in links. Way back Google posted an entry where they tried to clarify their nofollow attribute. However, the real picture is still not clear.
Adam Lasnik says:
> Does Google crawl a rel="NOFOLLOW" tagged link and not give it credit,
> or does it just stop at the link and not visit that page unless it's found elsewhere?
Sometime back Matt Cutts also informed that Google does not crawl nofollow links
At a link level, you can add a nofollow tag on the granularity of individual links to prevent Googlebot from crawling individual links (you could also make the link redirect through a page that is forbidden by robots.txt). Bear in mind that if other pages link to a url, Googlebot may find the url through those other paths.
Search Engine Watch has a good post on it.