A new app is available in the app store today called TextLater. It lets you set a date/time for text messages to be sent, so you can type them ahead of time and it will automatically send them later. This could be used for sending reminders to yourself, scheduling a late-night text message to send in the morning, and many other things. Here is the App Store URL: http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=289637800&mt=8
my opinion As for me, no matter at what time you sends a message to your girlfriend, the main thing - to please her. For example, there is the cool resource for cute good night messages http://sweetytextmessages.com/. Now to please the girl became even easier.