Steve,
As Ian points out, the automatic cleaning cycles are an ultra-sonic cleaning. This simply means that the sensor vibrates at a very high rate of speed in an attempt to dislodge any particles on the sensor (ok, again, I'm saying sensor out of convenience here, it is really the low pass filter in front of the sensor, but you get the idea). So get something a little more serious than a fleck of dust or such, and you sometimes do have to wet clean. And to answer your question - most modern cameras do have an auto-clean cycle these days... but my older Rebel XT (not much older than your XSi) does not.
This also can be a question of how you're shooting - you may just not be seeing the dirt. In my case, I started working with the Canon MP-E65mm macro lens that goes up to 5x magnification. At high focal lengths and or very small apertures (ie: f/16 and higher), the dirt on a sensor becomes more apparent. In the case of the MP-E65, I only really noticed the dirt at higher magnification - at 5x magnification, that lens can be stopped down to the equivalent of f/96, so any dirt becomes very visible. Additionally, depending on your subject, you may simply be missing the dirt in your photos - in my case, I was shooting tiny bugs on a white background, so it was easy to spot... but if you're shooting wide-angle landscapes, where you have a low focal length plus a very busy scene, it is easy to miss any dirt.
All in all, it is up to you and the results you're getting with your photos as to whether you feel you need to clean your sensor.
If you're interested, check out the blog post I made when I did my first sensor cleanings - http://www.ktuli.com/photography/node/353.
Also, my technique for testing my sensors is to use my longest lens, zoomed to the longest focal length, stopped down to the smallest aperture (highest f/#), set to manual focus so you can intentionally defocus the shot, then point at some fairly bright uniform surface, then as you are taking the shot (hopefully your shutterspeed will be sufficiently high) move the camera intentionally to ensure that anything in the frame is blurred (the dirt on the sensor is not affected by this movement, and therefor will stand out), then review the photo at 100%.
Hope this helps!
- Bill