I don't think you need the permission of these sites at all, don't waste time asking for permission. As an example, the app we have on the App Store (ScanBizCards) refers users to a number of web sites for information about recommended accessory. In our category other apps do a LinkedIn look up for contacts - again doubtful these app need LinkedIn's permission. Just one thing: you may want to ask for permission if the icon you present for these links is the site's trademarked logo - but even so sites are usually fine with their logo used in that manner.
and Evan, the internet is not "free". You still pay for it, unless you go to a public wi-fi hotspot, but bouncing off your neighbors is kind of rude.
Originally Posted by patrickq
Just one thing: you may want to ask for permission if the icon you present for these links is the site's trademarked logo - but even so sites are usually fine with their logo used in that manner.
+1 Same if you use a trademarked slogon or catch prase.
Now if you created a custom icon then it should be fine, but the icon shouldnt look at all close to the actual site logo.