It all depends on how you are with your phones. I've owned the iPhone 3G and the 3GS and I never accidentally broke either one. Beyond that I've only ever broke 1 phone I've ever owned by accident. I don't know how much the insurance is a month because I haven't looked into it yet, but lets put this into perspective:
1. The iPhone 4 costs $200 for the 16GB and $300 for the 32GB with a new 2 year contract.
2. If you aren't eligible for an upgrade at all and have to pay full price for the iPhone it's $600 for the 16GB and $700 for the 32GB.
3. After being in a contract for 1 year you get $200 off each phone if you sign another 2 year contract. But if you are an iPhone owner who's monthly bill is over $100 you get to upgrade to the new iPhone every year with the full upgrade pricing. If you are an iPhone owner who's bill is under $100 a month then I think, and I would check because I'm not 100% sure, that you get $300 off each phone, so it's $100 more than the full upgrade price.
I would take into consideration all these factors and how you are with cell phones before deciding whether or not to purchase the insurance. Now the insurance is always nice to have because accidents to happen, but you just have to decide if it's worth it for you.
Also, I've seen that there are other companies that sell insurance for the iPhone besides getting it through AT&T, you just have to be sure to get the insurance within 30 days of you purchasing the iPhone.
