Rosalind - 12:52 pm on May 13, 2011 (gmt 0)
I noticed Google mentioned spelling in its Panda suggestions. If that's the case you would tend to expect some forums to suffer a little compared with edited content, assuming edited content is a little more likely to get a spellcheck. But that really depends on the forum demographic, moderation level, and how picky the community is about grammar.
Then you look at YouTube's recent increases in traffic, and it's clear this can't be at all significant.