Hi there:
I don't know if you've made the decision on which camera to buy yet - but I'd like to make a suggestion if you are still seriously considering sticking with Canon.
There is a great camera review site
www.dpreview.com - and I got this side by side comparison here - the SD 900 you are thinking of, compared to Canon's S3-IS
http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_sd900%2Ccanon_s3is&show=all
Now, clearly there is a BIG difference in megapixels - the S3-IS is ONLY a 6MP camera, but unless you are planning on making posters, 6 is MORE than enough. Any 8x10 done with a good quality 6MP camera will look outstanding (as long as the shot wasn't done with digital zoom).
That brings up the next topic - zoom.
The S3-IS has a 12x optical zoom. I have the previous model Canon (S2-IS) which has the same zoom lens, but only 5MP and a couple less features. I cannot speak highly enough of the zoom feature of this camera. To give an example - when we were in Disney back in September, we went to the Animal Kingdom for the opening of the park. For any of you who have seen this, you know that while you are held back near the main entrance, Mickey makes an appearance at the Tree - but from that distance, he looks like an ant rather than a big mouse. Using the full optical zoom, we got a shot of Mickey that looks as though we were standing about 5' away from him. Additionally, "IS" stands for "Image Stabilization". On the long zooms, this is an absolute requirement to keep from getting a blurry shot - and the shot of Mickey was crystal clear. IS also helps considerably for normal shots without zoom - our previous camera didn't have IS, and we have noticed far fewer blurry shots with the IS.
You will also note that the S2-IS is just marginally cheaper, but this has more to do with the lower MP than anything else. It's also been on the market longer. With Christmas coming up, you should find some good deals.
Another point in favour of the S2-IS (although it might be a drawback, depending on what you want) - this camera "feels" more like a D-SLR. It is larger and heavier, but I find that I can support it better due to that. If, however, you want a camera that you can easily slip into a pocket - this is not the camera for you.
Other aspects that are nice - it is NOT a D-SLR, but allows you to grow in camera skills with some SLR-like functions. You can manually focus, set the f-stop or AP manually, etc. OR, you can just put it on Auto and use it like a point-and-shoot.
One warning, the LCD on the back is smaller than most newer cameras (only 2" as opposed to the 2.5" on the SD-900. HOWEVER, the screen flips out like on a digicam - allowing you to take shots of yourself perfectly framed, or hold it up high to shoot over people or obstacles. The screen can also be folded facing into the camera for protection - a VERY nice benefit.
In terms of the actual "eye" viewfinder - the SD-900 has the benefit of being an optical viewfinder, while the S3-IS is a small digital screen. Some people HATE this, but I have found the one on this camera to be decent.
One final point - the SD-900 has an internal battery with charger - the S3-IS uses 4AA (alkaline or rechargeable). If the SD-900 runs out of juice (I'm not completely positive on this, but I think I'm right), you are out of luck until you can recharge. Now, the S3-IS EATS alkaline (especially if using the flash), but if you invest in two good sets of NiMH rechargeables and a rapid charger, you should be fine. Should you EVER run out of juice with both - get a set of 4 alkalines, and your good to go for at least another 100 shots, even with using the flash for all of them.
Anyway, I just thought I'd put in a plug for my (admittedly updated version) camera. The biggest selling point is the zoom with IS. Go to a place with a demo model and just see how amazing the zoom really is.
Happy snapping!