Explaining "race" to kids

There’s a lot of talk now on DailyKos about race, how we need to talk about it, etc.

How do you talk to your kids about race?

So far (mine are 2 and 5) I’ve answered my older son’s questions pretty “scientifically” – some people have more melanin in their skin than others, etc.  Of course, that’s not the half of it here in the U.S.  
Right now all he knows really is that people come in different colors.  He is only barely aware that racism exists and plays happily with kids of all races/ethnicities in his kindergarten class.

It’s sweet to see the absolute lack of prejudice that that lack of knowledge entails, but at some point I’m going to have to start talking a little more clearly about racism, culture, and all those other more complicated issues.  I’m just not sure how yet.