Fiddling around on jsfiddle, it turns out that from jQuery 1.9 onwards, this problem does not occur: http://jsfiddle.net/wE577/2/.
A weird problem indeed. Unfortunately, my google foo didn't turn up any bug reports on this.
Since at the moment, we cannot upgrade jQuery beyond 1.8.3 (although I hope we soon will), a workaround I came up with is setting the placeholder
attribute after adding the new textarea
element to the DOM, which works flawlessly.