Right, it looks like unless I get a brand new computer/ operating system to test 3D Printscreen on, we've hit a dead end. When the software is running in the system tray, you can do your normal work and you'll be fine. When it tries to inject itself into 3d applications, you'll get two outcomes - either nothing happens, or it crashes. Sometimes it'll crash when you press the screen capture button, other times it'll crash on the rendering of the game. Bioshock 1&2 both fall into the category of "nothing happens". Source engine games crash when the hotkey is pressed, as well as the majority of other games such as DoWII, unreal engine games crash on render (and sometimes don't even render the mouse point for some reason), apart from UT3 itself which just feigns ignorance when you hammer the hotkey.
However, there is still light at the end of the tunnel - I've managed to get the software working for space rts games.....wooooh.....anyone want my 3dxml files for Empire at War and SoaSE? I thought not.
I don't know if it's steam blocking the injection, or not, so I'll have another fiddle with some other games I've got lying around.
Hope you've gotten more luck than me.