This example of stuff white people do is something that only some white people do. The number of white people who pet black people is limited to those few willing to get close enough to black people to touch them. However, this annoying, condescending behavior happens often enough to merit comment.
George Bush, Jr. is one white person who exhibits this trait, particularly the peculiar habit of rubbing black people's heads:
Although Bush is a fake Texan who actually grew up and went to school in the Northeast, he may be adopting an old white Southern custom here, that of rubbing black heads for good luck, especially those of children.* And Bush is not the only politician who does so. The practice seems to have spread North, where aptly named President of the Ohio Senate, Doug White, has also been called out for it.
Being petted by white people doesn't only happen to black men or children. As Nichelle at Anovelista points out, it happens even more often to black women. White women often admire the hair of Asian women, but there's something so fascinating about black women's hair that it sometimes makes white folks reach out and get personal. Notice how hard it is for Brandy and Tanika Ray to cover up their sudden discomfort when Barbara Walters can't resist playing with their hair:
A lot of white folks have other ways of treating black people like pets, but discussion of that form of racial condescension is enough to fill up another post.
*To be fair to Our Dear Leader of the Homeland, he seems to like touching not only black people; he has a more general bald-head fetish as well, and he has trouble keeping his hands off of people in other ways too.
UPDATE: For a first-person account from an African American perspective, see "Can I Touch Your Hair? Black Women and The Petting Zoo," at Womanist Musings, where Renee writes,
As a black girl growing in a mostly Greek and Italian neighbourhood, my hair often became the subject of conversation. I was a curiosity. People would touch it, and ask questions about its care like my hair was some kind of pet dog. That they were being racist, or treating me like some kind of exotic creature, never once occurred to them.