You are correct - The DRebXT's flash does not pop up in any manual modes (full manual, A-priority, S-priority, , etc.). It pops up in full auto mode (i.e. the green box). I haven't tried any other modes other than these in the last 3 years I have owned Rebels, so I can't say what it does in any other mode.
I like this feature. When I put the camera in full-auto mode (rare occasion but still happens), I want it to be completely auto, including determination of the need for flash. It is really helpful when I hand the camera to someone else, too!
But in fairness, on the slr's that feature the 'auto' pop-up flash,
am I correct in saying that the flash pops up and fires on its own
only in a fully automatic [green] mode or certain 'scene' modes ?
Sure hope it doesn't do that in manual, or in aperture or shutter
priority, or even in P (program).