Rebels are kinda cheap compared to their D series. The number series of cameras (like 20D, 30D, etc) are their prosumer/professional models. I have an Elan IIe (its a film camera though and their prosumer/professional model) and I love it. It takes fantastic photos and I bought it for 80 bucks and is a 10 year old model and still works flawlessly. If you do photography as a serious hobby, its worth it to look to their prosumer models. Otherwise if you are a hobbyist or casual picture taker, the Rebel series is more than enough. They use a lot of the same stuff as in their D series but the D series just has a bit more to do, but you more than likely won\'t use the features. I bought the Elan because 1) it was just as much used for the pro cameras as the consumer cameras for film and 2) they\'re more durable. But Canon and Pentax are the best in the business. If I didn\'t buy a Canon, I would have bought a Pentax. I bought a Canon because I was more than satisfied with their digital cameras and customer service and I\'ve used older model SLRs and they are typically ahead in the industry.
EDIT: I should mention too that the D series of cameras is double that of the Rebel. So unless you have the capital to afford it, the Rebels are fine
EDIT2: Before you buy the Rebel XTi, there is a new Rebel camera that is coming out in a couple weeks, the Rebel XSi. Its got the new Digic III processor and a higher megapixel count. I\'ve used Canon Powershots with both the old Digic II and Digic III processor. The Digic III is good but I personally really liked the quality of the pictures that the Digic II shot. I liked it so much, I\'m actually going to buy the same model camera (S400) that broke last year on eBay soon enough. I have the SD900 too and it takes fantastic photos as well. You won\'t notice a difference. I notice a slight difference in some shooting situations but if I gave you the same picture from the two different chips, I doubt you\'ll notice.
