Caroline,
I've been travelling with a Canon dSLR for several years now, so have a few ideas on what works for me. I started with the Rebel XT and upgraded to the 40D earlier this year. Which Lens to use will depend somewhat on what your intended use is? A few thoughts.....
The Rebel XSi is a good choice, but like any Camera, it has quirks. I always try to look at a variety of sources when evaluating new gear, but you may enjoy reading the XSi Review at DP Review. It received the coveted "Highly Recommended" rating. I'd also suggest reading the Forums for the XSi, as that's a good indication of user "real world" experiences.
One thing to keep in mind when buying a dSLR is that you're buying a system rather than just a Camera (which includes not only Lenses but also Batteries and other accessories from the manufacturer as well as others). Lenses are perhaps the most critical component of that system, and Canon has an excellent range to choose from.
You will probably find that you'll need a minimum of two Lenses. I usually travel with a wide angle, "walkabout" and zoom Lenses. The WA and "walkabout" get the most use BY FAR. One of the biggest advantages of an SLR is the ability to change Lenses, so if you would prefer not to do that then perhaps a "Superzoom" would be another option? While changing Lenses is a minor annoyance at times, it's sometimes necessary. The only alternative is to carry two Bodies, which is a greater annoyance IMO. If you want to check some "real world" opinions on Lenses, check the reviews at This Site.
You'll need to budget for at least one extra Battery, a robust Case and maybe a Filter or two (some believe in using UV Filters to protect the front element on the Lens).
There will be a learning curve with a dSLR, but I think you'll find that it's the best way to get great travel shots.