http://stackoverflow.com/questions/1111413/objective-c-determining-ip-address-of-ipod-touch-programmatically

gistfile1.m

#include <sys/types.h>
#include <ifaddrs.h>
#include <netinet/in.h>
#include <arpa/inet.h>
- (NSString *)getIPAddress 
{
    NSString *address = @"error";
    struct ifaddrs *interfaces = NULL;
    struct ifaddrs *temp_addr = NULL;
    int success = 0;
	
    // retrieve the current interfaces - returns 0 on success
    success = getifaddrs(&interfaces);
    if (success == 0)  
    {
        // Loop through linked list of interfaces
        temp_addr = interfaces;
        while(temp_addr != NULL)  
        {
			if(temp_addr->ifa_addr->sa_family == AF_INET)
			{
				if([[NSString stringWithUTF8String:temp_addr->ifa_name] isEqualToString:@"en0"])  
				{
					// Get NSString from C String
					address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr)];
				}
			}
			temp_addr = temp_addr->ifa_next;
        }
    }
	
    // Free memory
    freeifaddrs(interfaces); 
    return address; 
}