In dim lighting conditions, always use some form of AF assist. Be it a flash red AF assist light, our the camera’s flash AF assist, use it.

On the 40D, the focus confirmation will tell you the shot is in focus, however when examined afterwards on a computer, the short could have been sharper. This is especially true when shooting at wide apartures of under f2.0

I have ordered a laser pointer that continuously projects a pattern, and will strap it to the camera body to paint the subject with something that the AF sensor can track and focus reliably in low light, when the flash is not available.