Horizontal Case with belt clip
I found this case in a local Staples, but obviously is also available on-line:
Milante Black Lucca Case for MB810 DROID X ThunderBolt
Amazon.com: Milante Black Lucca Case for MB810 DROID X ThunderBolt: Electronics
w w w DOT a m a z o n DOT com/Milante-Black-Lucca-MB810-ThunderBolt/dp/B004E65W4U (sorry, too noob for links)
This works great because it is wide enough such that the top flap is not directly pressing on the power button.
I paid under 20 bucks at Staples, and obviously is cheaper online.