That is mysterious, Heather. Has the power company or anybody been pruning branches around your place lately? The wind could have blown some sawdust over - that's what it looks like to me.
But you can take a sample of it to your local Extension office, tell them where you found it, and they will have a microscope they can look at it with. That should determine if it's anything to worry about.
The other thing it resembles a little bit are Mosquito Bits. Any chance your County has been spreading those around your neighborhood lately? It's a good thing if they have, much better for you and the environment than spraying noxious insecticides.
