One hedge I always used to make was, "I think we have all the information we need for now. We'll get you an estimate within 3 business days."

And then I'd sit down and bust my ass trying to find out if it was feasible or not. :)

If it wasn't, we could always come back with a lesser approach and people seemed to be happy with that. ("It turns out Android doesn't widely support [feature x] so we recommend [feature y] instead.")