Another factor is simply price: the Curve on T-mobile (US) with equivalent voice/data service can be $20-$30 cheaper per month than iPhone on AT&T, even though the devices both retail for about $200 initially.
Very true. Best Buy is selling the 8330 (Verizon's Curve) for $39 this week (at least in NY); even on an off week, it's normally $100 there, which is a comparatively low barrier to entry into the smartphone market. I've seen 5 of my friends get Curves in the last 6 months.