Hi Dave
I've been thinking of this lens too, but I was worried that it wouldn't be wide enough for a cropped sensor like the 400D. Your 30D is cropped too isn't it - how do you get on? Do you find yourself using the 17-55 f2 a lot?
If I stretched to the 24-105L, I don't think I would be able to afford another lens, so I'd have to rely on my kit lens for the 18-24 range.
Yes, my 30d is cropped as well.
To be honest ive just sold my 24-105L because i wasnt using it any more. I tend to use the 17-55IS most of the time now. Its not mainly because of the wide end though, more because its really sharp and has f/2.8 and the pics i get from it never fail to make me smile when i see them on screen. In fact i really miss having 55-105mm but i now have a 70-200f/4 IS for that.
I also have a Tamron 17-50 f/2.8 and that is also a very nice, small lens. i found during the winter months i was using that a lot as it was faster than the 24-105L but i still missed the longer end so both did get used. when we went to Florida in Feb i got the 17-55IS because i only wanted to take one lens with me. I like having f/2.8 but also love IS so the 17-55Is is the best of both. I was surprised how good the Canon is and although i missed the extra reach i found i could crop in most of the time so its not really a problem for me. I do like to zoom more than shoot wide so for me even 24mm was fine but it is nice to have the extra mm on the wide end when needed.
It all depends on what you want to shoot but if you would be happy with the range of the kit lens then i would say you wont be disappointed with the Canon 17-55IS (or even the Tamron 17-50 f/2.8 if money is tight).
I think at the moment you have the whole range covered so what ever you get will make at least one lens redundant so i would make sure its the right range if you are going to spend out a lot of cash.
And from personal experience i would say get what you want now, even if it means saving for a bit. Ive gone through 3-4 cheaper lens's to get to the 24-105L. I should have just got it in the first place as it cost me more by selling the others.
Two other really nice cheap lens's to maybe consider are the Tamron 28-75 f/2.8 and Canon 28-105 USM MKII. This was my first upgrade lens and its a very nice, small lens that out lasted all my others.