I recently found this analysis and though it was worth a share also:
Nofollow in Google, Yahoo and MSN

To make a good and positive output in link building you can use no follow and do follow alternately so you can build natural inlinks for your site.

If you do link building with PR website, you got good positive output.

Google spiders always check no follow link, and they move on to the site that link is pointing. Only thing that is different from do follow links, is that link is not getting any value in SERP or PR and it's not making site any stronger.

No follow will definitely give you traffic but Google is never count as backlink to no follow links.

I think if you got backlinks from quality websites, nofollow ones can help you as dofollow ones. And Google count them as backlinks but as nofollow backlink which is less important comparing to dofollow ones.