You can’t get IMEI on iPhone anymore. You may have to use UDID instead. See other answers.
In the past, you could use the header file “Message/NetworkController.h” posted on ericasadun.com.
http://ericasadun.com/iPhoneDocs300/_network_controller_8h-source.html (It’s been removed now)
You would add the NetworkController.h file and the private framework “Message.framework” to your project, then import that header file to use the original method I found to get the imei number:
NetworkController *ntc = [NetworkController sharedInstance];
NSString *imeistring = [ntc IMEI];
That hack doesn’t work anymore. App will be rejected by Apple.