Just a few points;
1. There's no denying that in-lens IS is superior but Canons own recent marketing has indicated the advantage comes with an unacceptable price penalty. Here in the UK, the new 70-200/4 IS is more expensive than the non-IS version to the tune of a new Sony Alpha or 400D body. Thats on one lens and I think, clarifies my point exactly. Canon are using IS to screw us over blatantly.
2. Its possible Pentaxs' new 'three-dimensional' in-body IS will be highly effective and narrow the gap further. In body IS is newer and development continues rapidly.
3. Lens IS is especially effective on the longer and larger telephotos for which Canon already have perfect cover. Its a trivial matter to have the lens communicate to the body the presence of lens IS and have that take priority over in-body.
4. Who wouldn't want an instant IS collection comprising 24/1.4L, 35/1.4L, 50/1.2L, 85/1.2L and 135/2.0L? Lets face it, its staggeringly unlikely that Canon will ever release IS versions and if they did, you'd need the GDP of a small country to buy them given Canons recent pricing as a yardstick.
5. The current affordable IS stuff isn't great. As above shows, in-body IS makes the greatest IS compatible.
6. The in-lens IS range isn't growing very quickly and it could be years before many people get the lens they want, if at all.
7. Given how many people seem to want IS updates to lenses like the 300/4.0L and the 100-400L for example, its clear that costs will be never-ending for some. How much simpler it is to buy a single updated body every 2 years or so than replace four or five lenses. My 85L is 17 years old. How cool to have that usable with state of the art image stabilisation.
8. How long before market pressures force Canon and Nikon to implement it anyway? Lens IS may be better than in-body, but only if theres a choice to be made. In-body IS is infinitely better than no lens IS at all. Take Canons 400/5.6L as a perfect example.
I'm sorry, but these arguments that in-body IS aren't as effective as in-lens don't gel with me for these reasons. I mean, if its not available on the lenses I want, or the financial penalty is extortionate, where's the argument?
This feature above all others is going to be the one that cost Canon major market share unless it competes. Simple as that. Its just too compelling, especially at the lower, volume end and thats where many of the future Canon users start off.