Probably "tar spot" disease, which is a common fungal leaf disease of maples. It doesn't really hurt the tree, just looks bad. If you want to reduce the occurrence of this disease it is recommended to rake up fallen leaves and destroy them. Keep in mind that removing all leaf litter could reduce the accumulation of soil organic matter over time.

Aren't tar spots black (at least in the center)? The ones in the picture look almost red to me.

I'm thinking maybe it's maple anthracnose.

You might be right. Tar spot is typically black or dark brown. I figured the flash was causing the unusual coloration.

Thanks lycopus and brandon. The spots really were reddish and pretty bright. The color in the photo is awfully close to the real color.