Offline Maps for iOS4 has been released by iphoneofflinemaps.
A tip on how to install this is to use Maps Enhancer 1.3(not in Cydia). The reason why you should use it is Maps application will delete the caches of Google Maps when running Maps app without WiFi/3G. In other words, the cache data(Map data) may be deleted by Maps app even if Offline Maps replaces it with Map data that you wanna use. So, you may need Maps Enhancer to avoid this situation. The main function of Maps Enhancer is to keep the caches of Maps application.
