I won't be using the walkabout for sports so I'm not sure if I even need the 2.8. I am afraid to get an S lens since they are going to be outmoded as soon as the full frame models take over and if I am spending that much on a lens I want it to outlast this camera. The weight difference sounds good so I am tempted. I'll do a little more homework.
The 17-55 would leave me with a gap between 55mm-70mm range.
I am looking at this lens as an alternative, EF-S 17-85mm f/4-5.6 Image Stabilized USM.
I don't think you have to worry about Canon's APC sized sensors (1.6 crop) going anywhere for a long, long time. Having only two full frame models, it lends some justification as to why you would spend $8000 on their flagship camera (1Ds Mk II). If they moved all their bodies to full frame, they would have a hard time convincing pros that a slightly better Digic processor, AF, and weather sealing is worth $6000. This, coupled with the the fact that of all three sensor sizes that they offer, the 1.6 crop has been by far the most successful, and the concentration recently on only rolling out EF-s lenses (there hasn't been a new EF in a long time, only updates), I think Canon is convinced the 1.6 crop sensor is a HUGE potential market for them, the entry DSLR is where it is at right now, which means APC sensors, even for prosumer bodies.
As for the gap between 55-70, I don't think you will miss it at all. For a full frame, that is a gap between 88mm and 112mm. I currently have a gap from 85mm and 135mm, so a considerably wider gap than you will see, and I still don't feel the need to run out and get a 100mm, I don't even think about it.
As for the 17-85mm f/4-5.6, it all depends on what you want it for. If you are shooting outdoors or with flash, it will be a great lens. One of the interesting things about it though is that the majority of the zoom range is actually in the f/5.6 range. If I recall correctly, the stops are as follows:
17 mm - f/4
22 mm - f/4.5
24 mm - f/5.6
So at 24mm, you are already at the slow end of the lens. The IS is good for 2 to 3 stops, so you could conceivably hand hold this lens at f/5.6 with IS where you could do it with a f/2.8 or f/2.0 without IS, but it only helps you when your subject is static, so not great for people in moderate action. So, if you need 1/200 sec to get the shot with a f/2.8, you are going to need to go down to 1/50 sec with a f/5.6, which is a lot more time to get motion blur in your subject.
I always hate making these decisions, because one is almost twice the cost of the other, but you do get 4 times the amount light, therefor 4 times the speed. Really, it all comes down to what to do with the lens and what is important to you. For me, I am such a sucker for natural light and background blur, I almost always go for the faster lens, which almost always results in a dispute with the wifey over the cost
Now I need to upgrade my 70-200 f/4 but I'll save that for another day.
