Is there a way to strike out text in comments? The markdown that works for posts does not appear to work in comments.
In short, no. As Pekka pointed out, you can use unicode struck-out letters to give the appearance of a
<s> tag, but you can't use a proper strikeout. The SO Editing help that jmort253 linked in the comments explicitly enumerates all the markdown you can use:
_italic_ and **bold** text, inline `code in backticks`, and [basic links](http://example.com).
plus a number of shorthand links.