I'm the other way around, I have a custom built (by me) desktop with a lovely 24" Dell monitor, along with a late '07 Macbook.
To me, a PC is far more budget conscious than an iMac. Sure technology moves on, but can you really see monitors for example moving *that* far that you'd want a new one by the time a core i5/i7 became out of date? I doubt it. Unless by then we're all shooting 3D photos which I doubt
Regardless of whether monitors or any other component becomes out of date, the fact is that with Macs, when you need to buy a new one, you have to buy a new EVERYTHING.
This isn't very convenient in my opinion. Say, hypothetically, some revolutionary new port was coming out for computers in a couple of months (lets say USB666 for the sake of amusement xD) but you needed a new computer *now*. Well, if you bought your oh so pretty iMac right this second, where would you be when USB666 was released for PCs? Looks like you'd have to trade in your whole iMac for one with USB666 ports. And that's assuming that Apple had updated their line of products.
Now, on the other hand, what would a PC owner do? Why, buy a PCIe card with a few USB666 ports built in, job done.
I still don't buy the "iMacs are the best looking computer out there, hence to look professional you *must* have an iMac". Surely a photographer's work should speak for itself? Personally I wouldn't particularly want to work with someone who saw I didn't have an iMac and then decided that the lack of iMac meant I was no good. But then, that's just me, I take it you've all gathered I'm not particularly starry eyed for Apple products, I just happen to own a Macbook.