just to add a slightly different perspective, you should read "NurtureShock" by Po Bronson and Ashley Merryman, it's a very interesting read all about various studies that have been done on child development, many of which debunk a lot of our commonly held beliefs (and practices) surrounding parenting. there's a chapter in there on early development of race perception, racism, etc. that's pretty enlightening. it's entitled "Why White Parents Don't Talk About Race: Does teaching children about race and skin color make them better off or worse?" in (very) short, the findings are that the worst things we can do are to shush our child, subvert the conversation, fail to talk about it. (Same goes for all the other "embarrassing" stuff, like obesity, wheelchairs, etc. etc.) Shame is not the way to go. You just need to address it simply, kindly, frankly, and without apology. After all, the man IS black, and probably pretty aware of that fact. There's no offense in that. If you walked into a pizza parlor and a little kid said "Mommy! Look at that white woman!" you would probably just smile, be a little perplexed, and amuse yourself with the image of how the mom is going to handle that awkward little exchange..... Seriously, all you need to say (neither hushed nor overly loud) is, "Yes, honey, I see him." If the conversation stops there, great. If not, if he asks other questions, then just answer them.
It's true what people said about others not tending to take offense at the comments of small children, and you just need to trust in that. and if you do run into the rare person who takes offense, then you handle it by mentoring how you would want your child to handle it: you're polite, apologetic, but firm that there's been a misunderstanding and it's not the case that your child has been offensive, even as you're sorry that offense was taken. We tend to forget that all you have to do with small children is answer the question that was asked, and not turn it into a big conversation about sun exposure and genetic influences and race relations and where we come from globally and blah blah blah. Just answer the question that was asked, keep it light and easy, and chances are everyone will get on with their lives just fine. And -- don't underestimate the importance of this -- your kid will actually learn the "it's no big deal" lesson a lot more clearly than if you go on and on about how it shouldn't be a big deal and we shouldn't point things out and we shouldn't notice things that, frankly, we all notice. That's a very confusing message for a child. if it's really no big deal, then don't make a big deal out of it. the man is black. yep. he is. no big deal.
(ok, it took me about 18 hours to post my "slightly different perspective" -- kids derail my computer time, darnit! -- so, yeah, a couple of people weighed in in the meantime, which I hadn't seen.... in fact, the Vittrup study that Karen L. pointed you to is exactly the one that Bronson and Merryman used.... but heck, I still say what I say... :) )