Hi,
I also don't know a good deterrent.
Chipmunks (also called Ground Squirrels) LOVE birdfeed, too!
I have 3 birdfeeders in my back yard, with Wrens, Cardinals, BlueJays, and some Sparrows coming regularly for feed. They are beautiful. One is about 10 feet from my kitchen window, on the back porch, here in VA.
Here is a link to an "animal free" birdfeeder:
http://www.farmersmarketonline.com/cooperfeeders.htm
I don't know how much you might want to spend on one, but you can find others doing some research on Google, etc, typing in birdfeeders.
I do wish you good luck. PS; I also tried the Hot Red Pepper, crushed, and shaken over the bird feed. It doesn't harm birds, but is supposed to keep Chipmunks away, after they get some of it on their feet. Didn't work for me!