twitter/bootstrap — pull request #1957 by krainboltgreene (2012-02-13 21:30:45+00:00)
Title: Fix to be relative to the line height of the label

Found this when I had the lineheight set to 1.75em on a font-size of 18px.

The checkbox is scewed up by about 5px, so I changed it to em and it looks perfect.