When I just viewed this answer, I got two lines with "enter image description here" for almost 10 seconds. Then, it seems, imgur decided to provide the actual images. This happens from time to time, and it does annoy me a bit. Just for demonstration purposes, here's an image link with an invalid url , and with the default text ... (As Arjan points out in his comment below, this looks annoying with Firefox, but not with Chrome or Safari.)
I'd like to know how many people actually follow the request to enter an image description after uploading an image with the "image" button. Own research here on meta suggests that more than 50% do it, but "enter image description here" does show up quite a bit, and I guess the meta folks are rather educated in how the site is supposed to work.
My suggestion: Either change the text to something that would make a little more sense in a submitted post, e.g. "this should be the image description" (please improve!), or get rid of the default text altogether.
For some history: Until 23 Jan 2011, the text just was "alt text"; then Jeff changed it (trying to encourage people to enter an alt text, but doubting that it helps).