I just confirmed that this is the case... Here is a screenshot of the storm home screen with NO themes installed. Notice the verizon background is still there...
It is called the Blackberry Storm - Get with the program. Sometimes lightning occurs during a storm, thus sometimes you see the lightning wallpaper for the blackberry storm
Sorry all its not just Verizon. I rock a bell storm and from time to time I get the flash of lightning between screen changes. I love it and am happy to hear I am not the only one getting it. Yes you can make it wall paper but the lightning instead of fading is a unique thing and you have to wonder why it happens once every blue moon. There has to be someone who knows. Again its not the having bolts as a background we care about its the flash of them between screen changes we are all rattled by right?
Posted from my CrackBerry at wapforums.crackberry.com
LOL Usually when I hit icons on the homescreen I get chunks of the original theme lightning bolt pic to pop up where the icon is its weird wish it would stop though it gets annoying.
How do you get the storm to show with no theme? 
