Straight Talk SIM allows the iPhone and is $10 less for a similar plan with similar coverage. Their customer service is bad and their "unlimited" data may or may not have an unspecified limit near 2GB. Cricket and Virgin Mobile are also getting the iPhone, but they have poor coverage.
The next iPhone is expected in September or October, so the price will drop on the 4 and 4S. Use WiFi when you can. It's faster and will reduce your data usage.