Is it OK to drain sump water into a pond?
Unless you have reason to believe that the sump output is contaminated, and assuming that you have legal access to the pond, this is a perfectly viable solution. In fact, even if the pond does eventually (or occasionally) drain into a stream, the natural filtering effect of the process should remain somewhat intact. 

As Tester101 points out, local and state laws may apply. In my experience there's no problem ejecting groundwater into a wetland in a non-destructive manner. We've run open-loop geothermal systems into adjacent wetlands, and in Minnesota wetlands are strictly protected.