The dRebel inherited the 10D and SLR in general "professional roots", so the exposure approach is more conservative than the digicam ones, and it tries to keep the highlight info intact. This seems to be a good thing (and it is, if you postprocess the image) but the overall result is a darker picture. Digicams in general are less respectful of highlight info and they discard it to keep a more pleasing image, so they are better suited for P&S usage. To make things worse, in auto and scene modes you are unable to change the exp.compensation. You must use creative modes (as P, Tv or Av) to change it, so the dRebel is a "dark" auto camera.
In film, this is not a problem, because there is the additional photolab step, but in digital, if you don't postprocess, there is no "equalizer" step inbetween the phototaking and the resulting image, so if the camera took a dark image, yo get a dark picture.
Of course you can mimic the digicam "look" dialing positive exp.compensations (between 1/3 and 2/3 usually).
Flash exposure problems are a different beast. You must know how the flash meters the scene to obtain consistent results. My experience is flash always use some type of partial exposure metering, instead of evaluative one (I use center point to focus and metering) because I noticed the flash exposure is mainly calculated using the subject in the center point and not the whole picture. As the most tipical scenario is to have some subject brighter in the center than the background, the result usually is a darker than expected picture. I don't know if this is a design choice or a flaw, but knowing that fact, I always do FE lock in a gray area (and using AE lock I KNOW I'm using partial metering) and then focus and recompose the scene, and the results are pretty consistent.
The lack of in-camera FEC also makes the flash behaviour more annoying, but There is a free utility to change FEC in the Rebel/300D called FECset when connected to a PC via USB.
It can be downloaded from:
http://revolution.cx/rcx/fecset.htm