For all of those with a significant other and two Windows Phones, you'll want to check out Thinking of You by Quinn Damerell. Damerell is also the developer of the super popular Reddit app, Baconit, which just received a fantastic update.
Thinking of You is a secure personal messaging app the allows you keep in contact with your latest beau. The service which is free, works by connecting two Windows Phones via a personalized PIN. The service then pushes messages between two devices by connecting to a server for Toast and Push notifications.
It's cute, quaint, secure and free. The service doesn't maintain the messages on the server and get this, if you "break the connection" i.e. you get dumped, the app will automatically erase the messages on both devices. That's good news for those worried about their SMS messages coming back to haunt them later.
The app is fairly limited with just text being able to be sent (no photos, no location information) and you can only have one-secure connection at a time (sorry, swingers). It works well and there's even a prefab "Thinking of you!" message to quickly send to your loved one without all that effort making is a niche but well-done app.
If you have an SO that you want to use the app, just send 'em here to the Windows Phone Marketplace or the app's homepage for more info. Thinking of You is free and ad-free.