Transparency was on the AutoCAD wish list for many years before it was finally added into AutoCAD 2011. Transparency actually became an additional object property - much like color, linetype, etc that could be assigned to any object as well as any layer! Then it's simply a matter of setting the transparency level - anywhere from 0 to 90 (obviously you wouldn't want the value to go all the way to 100 - because that would make it invisible!)
Note; In order to display Transparencies - you need to have the Transparency toggle on!
Check out my latest Cadalyst video to learn all about Transparencies!
(and Yes, I am in mourning over the Super Bowl)