A few days ago, a hobbyist developer named keyboardP did a proof-of-concept trick using a modified PNG image as a lockscreen wallpaper for Windows Phones. The result is a transparent effect whereby you can see through to your homescreen. Kind of neat if you want to check the temperature without unlocking the phone.
The first one he did was simple: claw marks that made it look like someone had torn your screen.
Since then, members over at XDA have started to create many different variations, coming up with some real unique designs. Over at keyboardP's site, he has also accumulated a mini-gallery that is growing as users submit them.
Now, there are three things to know about doing this: one, you must email yourself the PNG file as syncing won't work (Zune only recognizes JPGs), two, the trick only works once you "touch" the screen and three, it won't work if you have your device password protected. Other than that though, it's pretty sweet and easy to do. You can see in the picture here that I went for a very simple all black that goes smoky when activated. Anyways, if you're bored, give it a go. Feel free to discuss and post your creation here in our forums.