If you're bored with the default lock screen in Windows Phone 7, here's how you can make it use album art to spice it up.
1. Go to the homescreen then tap Application Menu Key then Settings then Lock then Wallpaper.
2. Tap "show artist art when playing music."
This will show album art on the lock screen when you are playing music in the background.
[Windows Phone 7] Use album art as lock screen | Kioskea.net