I would refer you to section 3.3.2 of the iPhone SDK Agreement. It says "An Application may not itself install or launch other executable code by any means..."
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_0 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7A312g Safari/528.16)
You can link to the App Store if that's what you're trying to do.