Bing published a new blog post recently that highlighted their stance on poor grammar:
http://www.bing.com/blogs/site_blogs...u-have-it.aspx
They basically are saying, why would they want to show SERPs with all sorts of obvious errors? And if a site has a history of typos and other errors, that it's ultimately going to weigh in on your ability to show up high in the SERPs at all. We know that the engines are pretty good at spotting misspellings and suggesting the corrections - but overall grammar seems a little trickier for them to auto detect with accuracy.
I have mixed feelings on this personally though. It really depends on the format of your web site. If you are running a professional blog with just you or perhaps less than a handful of authors in total - I could see being harder with the site and the authors on grammatical errors. But if you are running an active forum or a site with a lot of activity in the comments section from visitors - it's far less realistic to expect the site owners to spell check everyone's comments - there are just far to many individual users to keep an eye on. People frequently abbreviate, or talk in slang or make special symbols out of characters. These are all technically "incorrect" too.
In the past, Google has kind of addressed this too by saying that there is a "correlation" between bad grammar and SERP rankings, but that they didn't have them directly factored into their algorithms at the time.
