I do on-site computer service and consulting for a living, and a good friend of mine worked for an extended warranty company for years, covering mainly computers and peripherals.
From everything I've heard and observed, I'd say that extended warranties are *typically* not worth buying. (For example, one of the major retail electronics and appliance stores in the U.S. was *only* showing a net profit last year because of money made on extended warranty and service contract sales. Without these, they would have been operating at a loss!)
Extended warranties should probably be looked at just like insurance policies for specific products. Not all insurance policies are created equal, and the same goes for extended warranties. Just because the store you bought a product from is "reputable" doesn't mean their extended warranties are too. You have to do a little research. EG. Years ago, I purchased an inexpensive HP color inkjet printer from the local Office Depot for my employer. Normally, this would have just been a "disposable" printer, *but*, we had already gone through 3 of them in 3 years. It printed color charts and graphs from a piece of expensive industrial equipment in a factory environment. Knowing that odds were VERY good we'd only get about 1 year of useful life out of this replacement, I opted to pay Office Depot about $45 for a 3 year warranty extension with "guaranteed replacement". Sure enough, after a little over a year, I got a call to "fix or replace the printer" again! I dug up my paperwork and called the toll-free number for the extended warranty. Found out it was handled by a 3rd. party which had recently gone bankrupt, and all claims were now being handled by a new warranty company at a different number. Called the new number, and they claimed they didn't have my warranty in their database at all! Went through weeks of hassle, faxing paperwork to people and so on. After all that, the final resolution was a compromise. The new company offered me $50 cash, via a compensation check in the mail, to "buy me out" of the remainder of the warranty. They refused to pay for the full cost of another printer!
Another factor to consider is the physical size of the product. On a very large product (such as a big-screen TV), the original warranty often requires shipping it back to the manufacturer for service. Obviously, FEW people are really ever going to do this. So effectively, you have a useless warranty unless/until you buy someone's extended version that includes on-site service if it's needed.
For what it's worth, I do know that with most higher-end digital cameras (such as your Rebel XT), the manufacturer insists that all repair work be done through them for a certain amount of time. (They enforce this rule primarily so they can get a first-hand look at what is breaking on their products to see if they can spot trends that would lead them to redesign things on newer models down the road.) This policy recently frustrated a buddy of mine who was primarily concerned with getting his camera repaired and returned to him *quickly*. I think he paid a local shop to fix his, rather than mail it back to Canon under his warranty, just so he could get it back in a "couple days or so". Turns out, the local shop just mailed it off to Canon anyway.