Doubtful their is somehting wrong with the camera - and these are my observations after hundreds of hours of editing RAW files from both the XTi and XT.
Figure it this way - the XT was 8 megapixels and the XTi is 10 megapixels.
All 10 megapixels fit onto the same size sensor meaning smaller pixels in the same space which in turn results into more noise. Now Canon knew this coming into engineering the camera. They actually implemented new noise reduction algorithyms to alleviate this inherent problem. I woudl say they did a pretty good job, but like I said, the original XT does much better at ISO 800. At ISO 400 their is less of a difference. It was my biggest worry when purchasing it and after reading enough reviews of people complementing Canon on fitting these pixels into their sensor, I took the plunge.
Overall their are other pluses to the XTi of course including the most importan to me whihc is 9 point auto focus, however it is no where near the same league as the 30D or the 5D in terms of ISO 800 noise reduction. Sorry but it is the truth.