Interesting colors you got. 30 seconds is not long considering. I have a #87 IR filter which blocks all of the visible spectrum, and it took a 5 minute exposure at f/8 in broad daylight on my 20D to get a correct exposure.
The Digital Rebel is more sensitive than the 20D to IR most certainly because the 20D has a stronger IR blocking filter of the cmos. It helps to reduce noise levels, which is why the 20D handles noise even better in the higher ISOs. I'm willing to bed the Rebel XT has this same stronger filter. It won't stop you from taking infrared photos, but it does make your exposures longer.