I wish that you had posted about you wanting a mantis shrimp a week ago! A friend of mine received about 100 LBS of live rock through mail order last month, and after setting it up in his tank (acrylic 125 gallon) he began hearing clicking noises at night. He couldn't figure out what the deal was, so I directed him to the thread that fishiebusiness posted with the creepy pictures of the mantis. Sure enough, after watching the tank for a couple of days, he found a 1.5 inch mantis shrimp rearranging his LR and feasting on inverts within the tank.
He captured it with a QT box loaded with tiger prawn meat, and traded it to the LFS for a cleaner shrimp. I got to see it briefly...it was quite beautiful, but also quite angry!
If you are really interested in mantis shrimp, the Discovery Channel did an hour long feature about them. It airs sometimes on cable, but is also available in the Discovery Channel Store (I'm sure its online as well). I forget the title, but I watched about half of it, and I just remember being awed by the power of the mantis' punch. The feature said that the power of their punch has been measured as equal to the impact of a .22 calibur bullet!