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.
Thank you a4mula, for instance the Radeon HD 3200 and 4350 only have dp support but not integrated support. So we can get DP with an active adapter. I was trying to find one passive adapter but with no success.