In contrast to the opinion of many here, I'm actually aiming for a 17-55 + 24-105 combo. I've had the 17-55 on order for a little while now, but I'm still not sure which is the better one to get first. The range of that EF-S, combined with f/2.8 and IS make it a killer for my indoor work. However I also do alot of 'half/whole day outdoor treks' which suit the 24-105 absolutely perfectly. I often need range more than 55mm but nothing THAT tele. Hence, the 24-105 matches perfectly. Otherwise I would have to swap over with the 70-200 constantly.
I'll probably end up getting the 17-55 first and then the 24-105 eventually. If Canon comes out with a 24-90-ish f/2.8L IS earlier, then I'll probably get that instead. Either way, I'am totally 'in' with the whole 17-55 24-105 combo idea.. but that's just me. 