I'd try to create a big stink about it in the press ("BIGCORP steals indie company's idea!!"), but not actually sue them or anything. This would have the benefi

I'd try to create a big stink about it in the press ("BIGCORP steals indie company's idea!!"), but not actually sue them or anything. This would have the benefit that people would hear about your product, which would benefit you both (though I assume you more because you'll have higher SEO and ranking in the App Store?).