Yes, I learned something new today. All the while, most of the Android builds that I tested came along with Superuser application by xda-developer, ChainsDD. And I did not bother what are its functions. I tried Moto-Mytouch 4G V0.4 alpha Android build. It was rooted but I cannot take screenshot with Screenshot application due to no root access permission. Huh? That build is rooted but I failed to access root. So I installed Superuser application from Android Market. Granted Screenshot application access to root. And it is working like a charm now.

Superuser for Android