Back in the film days, I'd say you have a scratched negative. But that's hardly the case.
First, it could be an issue with with the Aperture program. Try opening the JPG with the Windows Photo Viewer, or any other program that can open JPGs. If the stripe disappears, it's Aperture.
View the JPG on another computer. Perhaps it's the monitor. Back in the CRT monitor days, there was a horizontal internal wire on some CRTs that essentially prevented it from becoming oval shaped (screen physically getting wider). It would only be visible against very light colored displays. If the stripe doesn't show up on another computer, then it's the monitor and/or video card on your computer.
Lastly, if the stripe appears in the same location in all your photos, it may be camera related. Could be the memory card. Try a different memory card. If not, try shooting in RAW mode and using a program to convert the RAW to JPG on your computer. If the stripe goes away, then it could be the internal firmware in your XTi that needs updating.
If all else fails and you're getting that stripe all the time, sad to say, it may be send it to Canon time. It would also make good fodder for the Canon discount customer program where you send in an old Canon and get a discount on a new/refurb camera (I forgot what the formal name is)