I am seeing them in advertising but I can't recall a Black man, White woman couple among TV show lead characters yet. Black male, Asian females, or Asian and Latino males with others but that last frontier of the descendant of the former slave with the master's daughter has not been crossed on a major network I think.
Oh, there are plenty of examples. Let's start with Eriq LaSalle and Alex Kingston on
ER, fully a decade ago. In fact, LaSalle complained about having his character paired with a white woman, because of some sort of "message" he imagined it was sending.
More recently, there was DL and Niki on
Heroes, a married couple with a biracial son. Race was never an issue there at all. There's also Eddie Steeples and Jaime Pressly on
My Name is Earl. Apparently Marg Helgenberger and Gary Dourdan on
CSI had a borderline romance between their characters for a while. I also find a reference to a pairing between characters named "Skills" and Bevin in
One Tree Hill.
Beyond "major networks," there's Christopher Judge with both Musetta Vander and Jolene Blalock on
Stargate SG-1. The animated
Secret Saturdays on Cartoon Network is about a family consisting of a black man, a white woman, and their biracial son (though he looks more Latino or something, because of his freaky anime hair).
Pretty much every black male lead on
Star Trek has had at least one romantic plotline opposite a white actress: LeVar Burton with Susan Gibney; Michael Dorn with Suzie Plakson, Jennifer Gatti, Terry Farrell, and Nicole deBoer; Avery Brooks with Nana Visitor and Farrell (in the Mirror Universe only); Cirroc Lofton with Jill Sayre; Tim Russ with Sandra Nelson and Lori Petty; Anthony Montgomery with Johanna Watts.
More, from a 2006 article:
http://www.signonsandiego.com/uniontrib/20060215/news_1c15couples.html
Meanwhile, the forthcoming WB drama “The Bedford Diaries” features scenes of a black college freshman struggling through a relationship with a pregnant white girl still in high school, and an FX drama called “Thief” places Emmy-winning actor Andre Braugher in a marriage with a white woman.