As far as the theme having nothing to do with the admin area, I don't know that it's strictly true. Themes do add options, and for all I know they may modify existing ones as well. The problem seems to be one of timing, since you can almost guarantee the error by writing a post, pausing long enough for it to "autosave" and then trying to save or publish it... pooof! Gone.
I have several WP blogs on two linux boxes, and I am fairly certain that it is only the one blog that demonstrates this error. However, as I said, if you don't let it get the the "autosave", everything's fine and dandy.
I don't suppose you have any insight on how to sh!itcan the autosave feature, do you?
On my problem blog, I found the line a few minutes before you posted, but on line 1259, not 1257. My wp-includes/functions.php is dated 5/12 at 1737:
$trans['update']['post'] = array(__('Are you sure you want to edit this post: "%s"?'), 'get_the_title');
Unfortunately, it's not much help, yet, as it's a line that's just filling in an array of questions. Now... since you found that one so easily... where is that question used?
I'm looking for trans & update & post, but haven't found it yet... perhaps I'm looking in the wrong way? I'm heading over to the js functions to look - unfortunately I'm not much of a hand with the linux utils, so I don't know how to use anything but grep, and that not very well... it would be nice to have something like grep that will recurse subdirs... it might even be grep itself, huh?