Google's Matt Cutts stated that links with the nofollow attribute apparently do not enable more PageRank (or link juice) to flow to the followed links on the page. Instead, the PageRank that would have otherwise been passed just evaporates.

Pagerank evaporation occurs due to nofollow links on a page.
The nofollow links are not passing PR, so some of the PR will evaporate, but a part of it will still be distributed amongst the other outlinks.

many forums are nofollow - easily checked for those of you doing seo work... (like the op)

I come across this new word here and nice to know.

Actually it means to drop site ranking. Usually it is done if more no follow links are used than dofollow as nofollow doesn't provide any link juice and follow the links on the page. Which results in decrementing PR.

Changes in page rank of a website is known as PageRank Evaporation, i have heard this term very less in SEO field

Pagerank evaporation occurs due to nofollow links on a page.
The nofollow links are not passing PR, so some of the PR will evaporate, but a part of it will still be distributed amongst the other outlinks.

It's due to change in google's algo.

Yeah, i agree.