Canon's refurb program is good and offers some nice savings. Even better, if you can go through Canon Loyalty Program, trading in an old camera, that can add up to even more savings. Some poeple buy broken, cheap point n shoots that qualify for the CLP trade in off eBay, etc. You'll have to call Canon to see what's available through CLP and what trade-ins they are accepting, get a quote for your XSi. But assuming it's still working fine, you likely wouldn't get as much trade-in for your XSi, as you could sell it for here on POTN through the classifieds, on eBay or on Craigslist. For CLP, you generally want the cheapest, broken camera that qualifies, if you can lay your hands on one!
With either, Canon stands behind it with a 90 day warranty. If you have a problem, they'll make it right.
One thing, some of the major retailers also get refurbs to sell... Adorama and B&H sometimes have them. Their prices are similar to Canon's, but sometimes they often offer a longer warranty period. I've seen up to a year warranty on refurbs from those retailers. It might pay to shop around and look in the major retailers' used departments, too.
I have bought refurb lenses and cameras an never had any problem with them.
They might be returns for some reason or another, that were fixed. Or they may have been demo units in a store or at a trade show. Either way, they supposedly go through the Canon Service Dept. to be checked and probably get better/closer attention than a new camera coming off the factory assembly line that goes through a quick quality control inspection.