Vanilla 1.1.9 is a product of Lussumo. More Information: Documentation, Community Support.
We can't change the software, but I am told that once we move to the stack exchange platform, we'll have this preview feature.
Until then, you could try either making your comment in a text editor and then pasting it in, or you could delete your comment and make a new comment.
@Todd: Stack Exchange sites do have previews for answers, just like MathOverflow already has, but they do not (currently) have a preview feature for comments. This is fine; I think answers gain a lot more from having a preview feature than comments would. After all, edits to answers bump the thread to the front page, and the formatting options available in comments are extremely limited and simple compared to those in answers (apart from MathJax, but that is only present on a few of the sites in the Stack Exchange network).
@jdaw1: The edit feature for comments, which MathOverflow will obtain once it moves to the Stack Exchange network, is limited to 5 minutes. This meta.SO post provides more information about comments.
Ah, okay -- I wrote 'preview' but I think I meant there's a feature where you can edit comments. Thanks for the info, Zev.
1 to 5 of 5