Wallpaper Labeler Free and gCalWall Lite (direct links), both free and offering a $1 paid upgrade, work in similar fashion. By adding your own text with Wallpaper Labeler, or pulling down your next set of appointments from Google Calendar via gCalWall, you can alter an image or blank space to leave yourself notes, keep you on schedule, offer an "If found" reward, or otherwise give yourself something to peek at when you turn on your device.
Here's a few screens demonstrating the apps' abilities (click for larger images and descriptions):
Those aren't the only two wallpaper apps in the App Store, but they are straightforward, simple, and, well, free. Share your favorite tweaks for the "Touch to Unlock" screen in the comments.