I've always thought that the only way to get a partially transparent terminal window was to go to the command line: you know, 'defaults write com.apple.terminal TerminalOpaqueness 0.85,' or whatever it is. Then I discovered in OS X 10.1, that when you change the color of terminal windows, you reset the opagueness to 1.0 again. I got tired of having to reset the terminal opaqueness from the command line, so I did a little exploring. I reasoned that there should be a way to edit the Terminal preferences file using the Property List editor (hey, it was an inspired guess).
Sure enough, it works. Here's how: go to ~/Library/Preferences, and find the file 'com.apple.Terminal.plist' Double-click on the file, which will open it in Property List Editor. Click on the disclosure triangle where it says 'root.' Scroll down until you find the line that says 'TerminalOpaqueness.' If the terminal is not transparent, the value will be 1.00000 (I think you get five decimal places precision). Double-click the value, enter what you want it to be (I find a value around 0.75 to be good, although a lot of people recommend 0.85), and then save the file. Voila - your next open terminal window will be partially transparent!