In many applications, such as the Mail app, if the device is in airplane mode a notification will pop up that says \"Turn Off Airplane Mode or Use Wi-Fi to Access Data.\"
You could use the apple's reachablity framework when you app launches to check for network connectivity. Check this out
http://developer.apple.com/library/ios/#samplecode/Reachability/Introduction/Intro.html
I have a wrapper class for reachbiltiy APIs to make it simpler to use. (ASIHTTPRequest has one as well).
https://github.com/daltoniam/GPHTTPRequest
see the GPReachablity class for checking for connectivity. As far as a dialog prompt, not sure there is a way to push to the settings app. Any questions let me know.