Are you happy with the pictures you're getting now? If the answer is "Yes", save your money. If the answer's "No", then why aren't you? If it's not because they're blurry from camera shake, you don't need IS. The time to get a new camera (says he, hiding his signature, which is out of date) is when you need (or think you have a good use for) a new feature it introduces. There'll be another camera along next year, and the year after, and . . . Sooner or later one will come along that offers a feature you really do need. Maybe it is the SD1100, but maybe it isn't.
I bought my D60 in late 2002. Two months later the 10D shipped. It had a bunch of new features, and cost less than the D60 had. I would have liked the extra AF points and bigger buffer (more shots in a burst), but at that point I didn't really need them. By the time the 20D came along, however, with even more AF points and bigger buffer, I knew I did need those features. So then was the time to upgrade.