I'd guess the obvious question here is what video cards are you referring to. From the 5770 up to the 5970, all of the ATI cards have DP out (5970 is mini-dp, as is 5870 eyefinity 6). I can't speak for Nvidia's newest offerings.
Any card can support DP through adapters, so it might just be misleading advertising as well. DVI > DP, VGA > DP adapters are floating around, and most monitors that ship with displayport come with these adapters.