Lame_Wolf - 2:33 pm on Nov 15, 2012 (gmt 0)

Or might you end up with a 600 pix deep box with one normal size text ad in it?

Yep, that is exactly what happens. I've just spotted one with just one advert in it, and a huuuuuuuuge space underneath.

(There are enough advertisers to fill those gaps many times over, so lack of inventory is not the reason.)