Wirefly and LetsTalk just resell service for Cingular, Verizon, Sprint, T-Mobile, and Alltel. Usually they will sell their phones cheaper than what the carriers offer the phone. They can do this and still profit because of the 'bounty' they get for bringing in new subscribers.
For anybody concerned with problems that may arise, if it's possible you may want a refund, if you want less of a hassle with a mail-in rebate, or basically any other customer service issue that may arise, it may be best to pay a little extra and buy directly from the provider.
Also note, sometimes Letstalk and Wirefly don't offer the full 15 or 30 day trial period. Sometimes it's just a 7 day trial and that 7 days starts when the package is mailed. There have been a lot of unhappy Letstalk and Wirefly customers but if you know exactly what you want and you're not wishy-washy and don't expect much customer service, it can be a good way to save some $$$.