keyboardP a windows phone 7 developer has just uncovered that you can use Transparent images as wallpapers on Windows Phone 7 lock screen. It looks quite nice , now you can take a quick glance at your home screen without unlocking your device. Installing these wallpapers is a bit tricky coz if you try to sync the wallpaper via the zune app it will convert the PNG image to JPG and so all the transparency is lost :( , but there is a way around it.
Here is what keyboardP had to say about it
I decided to try and email myself the image as a PNG and download it via the phone. This seemed to work perfectly. Simply send yourself an email with the image attached, then open the email on your phone and save the image. When you view the image, you have an option of setting it as your wallpaper.
NOTE : This will not work if you have a password lock for the lock screen.