1. Check if the user-agent matches an iPhone/iPod Touch
2. Check for an appInstalled cookie
3. If the cookie exists and is set to true, set window.location to your-uri:// (or do the redirect server side)
4. If the cookie doesn't exist, open a “Did you know this site has an iPhone application?” dialog with a “Yes, already got it”, “No, let's try it” or a “Leave me alone” button.