I believe that that is only possible in .Net applications. It is probably some setting in the form's properties.
I use Visual Studio 2005, and in that version I am unable to make any part of the background of a form invisible.
I am, however, able to set the borders of the form to whatever shape I want. It might be possible to set up some advanced border that only includes 50% of the pixels in a set region.
Also, I believe this feature is only available in Vista (or perhaps the .Net framework 3.0). I have not used Vista extensively, but do all programs have transparent borders and title bars, or only certain MicroSoft software?
Do you understand what I mean? I'm terrible at explaining things. :-P Ask me to clarify if needed.