Terminal windows can be made transparent simply by typing the following text at the command line:
defaults write com.apple.terminal TerminalOpaqueness '0.85'
The amount of transparency can be adjusted by using values between 0 and 1 (with 1 being completely opaque).
You will need to open a new terminal window to see the result.
You can set _all_ your terminal windows as transparent by using Terminal > Window Settings, setting it the way you want it, and then pressing the "Use Settings As Default" button.
No, Zammy. Window ShadeX haxy does that 'kind of', but at least it didn't give the transparency as I'd have wanted it. You'd need to play with the core of the system and add there Alpha Level setting - after which you could play with it - in all Cocoa apps.