Martin,

I have an unlocked (by Apple) iPhone 3G, so I do not use AT&T. Rather I use t-mobile. I have four lines and pay about $120 - 130 per month including unlimited internet on the iPhone and 400 text messages per month each on two of the four lines. For that cost I get 1,000 minutes per month. Evenings, weekends and calls between t-mobile phones do not bite into your minute pool.

Going this route, you will pay more for the phone up front but lower ongoing usage.

Hey, if you still have a business, why not get it on your business dime, then you can write it off. (check with your CPA)

If you go unlocked, you want to make sure you get the official Apple unlocked phone (you'll have to get European or Asian phone) so that software upgrades do not lock your phone back to AT&T. The phones that are software unlocked will lock after an upgrade. And the 3.0 version is coming out real soon.

My monthly telecommunications costs per phone are pretty close to yours.