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.
T-mobile has absolutely NO coverage in and around where I live. Not even low signals - no signals.