There are some parents of kids who are much more advanced in math than my kiddo, but I too have found that it's difficult to find perfect placement (although I think we're hitting it close right now at a school for HG kids). I think Carolyn K on Hoagies came up with the concept of "least-worst" situation for these kids who are out of place in the standard school situation. I think that's all we can really do -- find options that will be the best fit for the current time/place/situation. Things will change, and although the school will want to throw out all the "what will we do when he gets to such-and-such grade," I think the best thing we can do is teach our kids the best we can right now. See Tamara Fisher's blog entry: This Year's Kid, Not Next Year's Teacher

Wren makes a great point about online options, which are flexible because they go at your own kid's pace. There are a lot of great one's out there. Davidson has a nice comparison chart: Online Math Program Comparison

Good luck!