There are several things you need to be aware of, firstly camera shake is largely dependant on what the shutter speed of the camera is, for example if your shutter is tripping (snapping a photo essentially) at 1/1000 sec, you're unlikely to encounter any camera shake, but as it gets slower like 1/30, then you'll run into problems. Ask the sales rep to demonstrate 1/30 @ 55mm with and without the IS on.
Generally, for shorter lenses like the 18-55, imo you don't really need the IS, I have shaky hands(Essential Tremors) fyi. But for long telephoto lenses a la 70-300, 55-250, 70-200 etc etc, at the long ends, you'll notice that without the image stabiliser things can get pretty shaky. For telephoto lenses, you'll definitely want the image stabiliser.
As for IS built into the bodies such as Sony DSLRs, I hear you can't see the difference through the viewfinder whether or not the image stabiliser is actually working. However for a lens-based IS system, when you half depress the shutter release, you'll notice your view automatically stabilise(try this at longer focal lengths for a clearer demonstration). This is comforting to know and as you progress would be good knowledge as to what shutter speed you require to grab the shot without camera shake coming in.
Finally, don't get the 1000D, with the 500D now out, the 450D replaces the 1000D as Canon's entry-level DSLR and eventually it will go out of production, but thats not my point. My point is that the 450D is a better camera in my opinion. My apologies to any 1000D owners if I have offended you in anyway:embrass: