Easiest way to detect Internet connection on iOS?

I did a little more research and I am updating my answer with a more current solution. I am not sure if you have already looked at it but there is a nice sample code provided by Apple.

Download the sample code here

Include the Reachability.h and Reachability.m files in your project. Take a look at ReachabilityAppDelegate.m to see an example on how to determine host reachability, reachability by WiFi, by WWAN etc. For a very simply check of network reachability, you can do something like this

Reachability *networkReachability = [Reachability reachabilityForInternetConnection];   
NetworkStatus networkStatus = [networkReachability currentReachabilityStatus];    
if (networkStatus == NotReachable) {        
    NSLog(@"There IS NO internet connection");        
} else {        
     NSLog(@"There IS internet connection");        
}

@BenjaminPiette's: Don't forget to add SystemConfiguration.framework to your project.

Seeing as this thread is the top google result for this type of question, I figured I would provide the solution that worked for me. I was already using AFNetworking, but searching didn't reveal how to accomplish this task with AFNetworking until midway through my project.

What you want is the AFNetworkingReachabilityManager.

// -- Start monitoring network reachability (globally available) -- //
[[AFNetworkReachabilityManager sharedManager] startMonitoring];

[[AFNetworkReachabilityManager sharedManager] setReachabilityStatusChangeBlock:^(AFNetworkReachabilityStatus status) {

    NSLog(@"Reachability changed: %@", AFStringFromNetworkReachabilityStatus(status));


    switch (status) {
        case AFNetworkReachabilityStatusReachableViaWWAN:
        case AFNetworkReachabilityStatusReachableViaWiFi:
            // -- Reachable -- //
            NSLog(@"Reachable");
            break;
        case AFNetworkReachabilityStatusNotReachable:
        default:
            // -- Not reachable -- //
            NSLog(@"Not Reachable");
            break;
    }

}];

You can also use the following to test reachability synchronously (once monitoring has started):

-(BOOL) isInternetReachable
{
    return [AFNetworkReachabilityManager sharedManager].reachable;
}