If you're worried about the size for vision reasons, remember that you very well might be looking at a notebook much closer than you would be if you were using a desktop/conventional monitor.
That's a very good point. Even on the High Resolution screen on my 17" MBP I find I can read everything pretty easily BUT I do have my MBP on my lap all the time so I'm pretty damn close to it.
You keep saying how you want a large screen; I assume that's because of the increased physical size rather than resolution, right? Cos I was in your very shoes 3 months ago when deciding between a 15/17" MBP or 24" iMac.
I went for the MBP because I liked the idea of having lots of windows open at once which the low resolution of the 15" just couldn't provide. It helped that the 17" had the same resolution as the 24" iMac and frankly, seeing the iMac in store now, everything just looks too big and nowhere as elegant as on the MBP.
My old PC upstairs has 3 monitors connected to it and yet I hardly ever use it anymore! I thought I would be splitting my work 50/50 between the PC and the MBP but my MBP is to me a portable desktop. Lounge, kitchen, garden, summer house, office. Try doing that with an iMac!
Another reason I chose the MBP was because it supported the 30" ACD but I STILL haven't bought it can you believe! I can do everything I need to on my MBP, especially thanks to Spaces, and be far more comfortable than having to sit in front of a desk.
If you want a TRUE replacement for the 24" iMac get the 17" MBP Hi-Res. But from what I've read you probably won't be able to afford it and it won't satisfy your desire for a physically large screen. In that case get the 15" MBP and a potential 23" ACD or 30" down the line.
For anyone else considering it, the 17" MBP is a serious contender, worthy of your consideration. It's like having my whole office on my lap wherever I go, and I'd never go back and I wouldn't be surprised if I never even get an external monitor for it!