Here is the press link release from ATT:
http://www.att.com/Common/aboutus/files/iphone/pricing_fact_sheetip4.pdf
No Commitment
Any customer who wants to purchase iPhone 4 without signing a new 2-year agreement may do so at the no commitment price. An eligible voice and data plan are required.
The 54.99 is just the Voice plan, you still have to purchase a Data plan. What happens is you must commit to buying at least 1 month of service in order for you to recieve the phone. It's like paying month to month where you can stop paying at anytime, but initially you MUST buy service.
Here is the link for the data plans available:
http://www.att.com/shop/wireless/plans/data-plans.jsp
The cheapest Data plan is $15 a month so in actuality, the initial payment for a locked, no commitment Iphone is $599 or $699 plus $54 for voice and $15 for data, plus sales tax.
Then if you wanted to illegally unlock it, expect to wait a few weeks and pay $25-40 for a service or program that does it. Not to mention then that whatever carrier you plan to use, you will need a new sim card, because they are not using the standard sim chip on these phones. Most likely, you will have to pay for that new sim card, plus whatever data and phone plan you plan to use. I am not sure the cost of a new micro-sim.
So the big question is, do you buy an official unlocked phone in Europe at full price (most likely to be around $1000 bucks, where you will have a Warranty, or spend $700 or $800 for one with/out the warranty - plus the price for a new sim and plan on a different carrier.