Hi, I recently intalled the firefox nofollow link detector: http://blogoscoped.com/archive/2005-01-19-n34.html From what I can see, most external links from blogs, youtube and wikipedia are all nofollow. I think I am right in thinking that this will not help your google rankings at all to spend time posting.
You're right. Links with noflollow won't help your search engine ranks. They still might be able to generate traffic from the link itself though so I wouldn't completely discount them if you can get the link in a good place.
I don't think that Google Page Rank is as accurate as it could or should be. I also don't think it really matters as it is not a true indicator of the quality of a page (though can assist in at a glance guesswork). I feel that when picking what sites to get the best link backs from Traffic should be the biggest concern. A link to a shared account on the front page of YAHOO can completely put the entire server at a standstill. Can you imagine how much traffic that would require? If you could handle it, then even if the link was nofollow, the number of conversions would have to make up for it...after all, it is ROI in the end isn't it? So personally I'd rather have 10 PR0 links that have good traffic then a single PR10 page link that no one ever bothers to visit.
one positive point about a wiki link, besides traffic, is the page strength value it adds. some people put a lot of stock in the seomoz page strength value if you are planning on selling your site.