It's a consideration isn't it? I think we need to remember that using digital has removed all the constrictions on film and cost and we tend to shoot many many more images than we would have done with film, so we shouldn't be surprised that shutters now take a severe battering in a shorter time than they used to.
Your 20D should be good for about 100K exposures according to Canon, so you shouldn't be too concerned. I don't want to sound like a smart arse here, but I don't think I've filled a 1 gig card up yet and I sometimes go out with two. You should have a pretty good idea when you press the shutter whether the shot is in the bag, and now of course we can review the shot on that handy LCD screen. Shooting off loads of 'maybe' shots in the hope of getting a good one is useful sometimes, but something to avoid doing on a regular basis.
Trying to be a little more selective about the shots you take, will probably do more for your photography than anything else anyway, but your 20D should be good for the equivalent of shooting a 36 exposure roll of film every day for around 8 or 9 years.... and that's a lot of shooting.
