The other day, Google's Matt Cutts posted a video answering if poor grammar in comments hurts the page's rankings. In short, the answer is no - bad or poor grammar does not hurt your rankings if done in comments.
Here is the video answer:
Now, Matt did add that you should not let spam comments through, implying that those may impact your rankings. He also implied that bad grammar that you write on your own site is a bad thing and you should make sure to use proper grammar. Back in 2011, Matt actually said grammar is not used in the ranking algorithm at all. He said this after Panda launched, by the way.
Here is that video:
Now he does go on to explain that reputable sites do spell better. He also mentioned the reading level feature they launched a while back.