fumanchu - 4:33 am on May 9, 2013 (gmt 0)
I've never seen 300x250's substituting for 336x280. That's a bit odd.
I gave up on the 336x280s though, as I had better luck with the 300x250 size. I don't know if that's because there are more advertisers or what. But I also like the fact that they take up less space on a page. Works better for me.