Make enough to afford a Pro, but too smart to waste the $$$...
Most "Pros" should be making plenty enough money to afford a Mac Pro, otherwise they are not "Pros", they are just aspiring.
When the 24" iMac came out, I took a chance and bought it over the Pro. I saved myself about 3-4k, and never regretted the decision once. My business associate appreciates the extra room created under my desk (he is a dog, lol).
Expandability is greatly overrated, the fact is most "pros" can only keep their machines for 3-4 years. I would much rather replace a 3k system than one that cost 7-10k (which was the average cost of most of my previous workstations). It was not much of a gamble, as I guessed that I could even buy a system every 2 years and still come out cheaper... not to mention my newer machine would be way more faster and powerful than whatever I was working on.
My first gen 24" is showing its age a bit, but is still as reliable & effective as the day I bought it. The fact is my needs have expanded beyond it, and my image library is now around 65-75,000 images. At 3gb of RAM I simply need more juice to keep Aperture happy and run as fast as it used to. I knew a bigger version was going to come out sooner or later (I was hoping for a 30") and will move to replace my work station with the new 27" iMac.
If you work for someone else, perhaps it doesn't matter how much your workstation costs, as it may only effect your raises/bonuses (or lack thereof). If you work for yourself and are successful at it, you will soon realize that EVERY dollar saved each year translates into a dollar of profit. It is the easiest money you will make I would wager as well.
Telling people that you are only a "pro" if you work on a Pro system is ridiculous. Unless someone is working professionally with video, the iMac is a wise and sensible choice for many "real" professionals. The myth of the pro for pros is one I am sure Apple is happy to continue with. I have maintained that the bigger iMacs occupy a pro or pro-sumer niche for awhile now. With 16gb of RAM, 2 TB of internal HD space, i7 Quad Core processing... I will be thrilled I waited even though I didn't get my wish for a 30". My 24" will still be in service as well, and am guessing I can still get another 3 years out it in another capacity.
The hoopla over the lack of matte options is blown out of proportion. Before flat panels came out ALL of my high-end were glossy CRTs and we coped just fine. I agonized over my big Samsung HD TV hanging over my fireplace as well, as it was one of the first ones to come out with the glossy screen. The fact is that when "off" and black... there is indeed a glare/reflection issue.... but when "on" the image really overwhelms the reflections and one does not notice it as much as you would think. Head to your local bigbox and try it for yourself... start turning a few glossy TVs off and see what happens.
I have no fears whatsoever about being able to effectively calibrate my new screen to match my output. They said the same thing about the white 24's when they first came out (that they were too bright to be able to be properly calibrated... and they were wrong about that as well).
My advice is max out the iMac as far as it can go, and you will be well served with a trusty workstation for the next 3-4 years. Take the money saved and take a trip, purchase a laptop, whatever... but you will be able to laugh all the way to the bank, just like I have done.
cheers, michael