As this answer shows, the editor icons of Stack Overflow [et al.] are packed into a sprite. To recapitulate, here is the image:
Lately the envelope icons, among others, have been changed. When looking into that, I noticed those icons weren't packed into sprites, but existed just as separate images, e.g.:
alt text http://sstatic.net/stackoverflow/img/replies-off.png replies-off.png
alt text http://sstatic.net/stackoverflow/img/replies-on.png replies-on.png
alt text http://sstatic.net/stackoverflow/img/comment-del.png comment-del.png
alt text http://sstatic.net/stackoverflow/img/comment-del-hover.png comment-del-hover.png
alt text http://sstatic.net/stackoverflow/img/feed-icon-small.png feed-icon-small.png
alt text http://sstatic.net/stackoverflow/img/vote-arrow-up.png vote-arrow-up.png
et cetera, et cetera.
According to a comment on this answer, doing multiple HTTP requests on a mobile device costs much more time than fetching one large(r) file in a single run. That way, Jon Skeet can make comments and edits on his travels to and from his work way more rapidly than in the current situation.
So, why aren't all icons packed into sprites?