We have had an unusually cool, wet spring and the maples here (Cincinnati)
are exhibiting sighs of fungal disease. Your tree probably has
anthracnose.
It shouldn't be fatal to the maple, although it will kill native dogwoods
(cornus florida). Here is a link with some pictures:

http://www.extension.umn.edu/project...nosemaple.html

Thanks Vox,

After looking at the pictures and the descriptions, I'd lean toward
Phyllosticta (Phyllosticta minima), because of the black dots that are clear
in the pictures I took. I also found this description of Phyllosticta from
another site
(http://www.glfc.cfs.nrcan.gc.ca/tree...af_spot_e.html):

Spots on infected leaves are brown with surrounding purple borders. They may
be circular or irregular in shape and 5-10 mm in diameter. Tiny black
fruiting bodies are scat- tered over the surface of the spots. Occasionally,
the dried-up central portions of the spots crack and fall out before the end
of the summer.

Here's another picture of Phyllosticta minima from another site that looks
very much like what my tree has:
http://maple.dnr.cornell.edu/images/diseases/dis4.jpg

John