I'm currently using the following to check whether Wi-Fi is available for my application:

#import <SystemConfiguration/SystemConfiguration.h>
static inline BOOL addressReachable(const struct sockaddr_in *hostAddress);

BOOL localWiFiAvailable()
{
    struct sockaddr_in localWifiAddress;
    bzero(&localWifiAddress, sizeof(localWifiAddress));
    localWifiAddress.sin_len = sizeof(localWifiAddress);
    localWifiAddress.sin_family = AF_INET;
    // IN_LINKLOCALNETNUM is defined in <netinet/in.h> as 169.254.0.0
    localWifiAddress.sin_addr.s_addr = htonl(IN_LINKLOCALNETNUM);

    return addressReachable(&localWifiAddress);
}

static inline BOOL addressReachable(const struct sockaddr_in *hostAddress)
{
    const SCNetworkReachabilityRef target =
          SCNetworkReachabilityCreateWithAddress(kCFAllocatorDefault,
                                                 (const struct sockaddr *)hostAddress);
    if (target != NULL)
    {
        SCNetworkReachabilityFlags flags = 0;
        const BOOL reachable = SCNetworkReachabilityGetFlags(target, &flags);
        CFRelease(target);
        return reachable && (flags & kSCNetworkFlagsReachable);
    }
    return NO;
}

This, however, does not return NO as it should when the iPhone is connected only to a cellular network but not a Wi-Fi network. Does anyone know how to fix this?

Edit

So this is what I ended up using:

#import <arpa/inet.h> // For AF_INET, etc.
#import <ifaddrs.h> // For getifaddrs()
#import <net/if.h> // For IFF_LOOPBACK

BOOL localWiFiAvailable()
{
    struct ifaddrs *addresses;
    struct ifaddrs *cursor;
    BOOL wiFiAvailable = NO;
    if (getifaddrs(&addresses) != 0) return NO;

    cursor = addresses;
    while (cursor != NULL) {
    	if (cursor -> ifa_addr -> sa_family == AF_INET
    		&& !(cursor -> ifa_flags & IFF_LOOPBACK)) // Ignore the loopback address
    	{
    		// Check for WiFi adapter
    		if (strcmp(cursor -> ifa_name, "en0") == 0) {
    			wiFiAvailable = YES;
    			break;
    		}
    	}
    	cursor = cursor -> ifa_next;
    }

    freeifaddrs(addresses);
    return wiFiAvailable;
}

Thanks "unforgiven" (and Matt Brown apparently).