Yeah, I have. It probably depends on the individual word cloud site, and how it has been setup. If they block robots or use rel="nofollow" then they probably won't appear.
I own a word cloud directory. Unlike others, it really has a directory structure for each category. Not one page site. http://www.word-tagcloud-dir.info
Yeah, I have backlinks from word cloud sites. One of the things worth checking when considering buying a link on a word cloud site is when it was last cached by Google. Check out this post and this list of top 100 word cloud sites on my blog.
I've put my link on a fair share of them and have never noticed them in the backlinks. And on particular page that i mainly just threw on word cloud type sites dropped form a PR4 to a pr0 which is very strange when parallel pages are at least pr3s or pr4s several subpages have pr5. If anything can be considered a "link farm" - a word clouds and pixel sites would be don't you think?
Hmmm haven't seen any yet .. but what's the worth of a link from a word cloud site anyway? I don't think its very high ..
the problem for typical pixel and word cloud sites got only one page, listing all kinds of websites. Now, that's a typical link farm. That's why I built categories to sort websites in my word cloud directory. Each page lists websites that have common theme.