Have you noticed that some sites in iOS safari will show a banner to let the user know the app is installed? How does this work? Is this a meta tag?
You should include
<meta name="apple-itunes-app" content="app-id=myAppStoreID, affiliate-data=myAffiliateData, app-argument=myURL"> in your webpage's head.
Please check this link for more information: http://developer.apple.com/library/ios/#documentation/AppleApplications/Reference/SafariWebContent/PromotingAppswithAppBanners/PromotingAppswithAppBanners.html
NOTE: It only works with iOS6 or later!