[General Questions and Answers] swazey:
Actually - it is against the law for them NOT to transfer your number. Since they already confirmed you could cancel your contract early, you're all set. If that was not the case, then you would have to pay an early termination fee (or whatever your contractual agreement is). Just make sure you get your current number "ported" to the new carrier before your contract officially ends.

This is directly from about.com:

"Question: Number Portability: Can I Transfer My Cell Phone Number?

Answer: Yes. In the United States, wireless local number portability (WLNP) is a legally mandated service that allows the transfer of a cell phone number from one carrier to another.

History

Number portability for landline phone numbers existed before it did for wireless numbers. In July 2002, the Federal Communications Commission (FCC) set a Nov. 2003 deadline for WLNP to take effect. Verizon Wireless resisted.

The FCC activated WLNP in Nov. 2003 in the top 100 metropolitan statistical areas (MSAs), which are the major cities in the U.S. In May 2004, the FCC made the service live in the rest of the U.S.

The FCC also made it so a landline number can be transferred to a cell phone carrier.

Overcoming Hurdles

Wireless local number portability has come a long way in the U.S. Transferring your cell phone number from one carrier to another used to be more complicated than it is today.

The switch also used to take longer than it does now. While the process of transferring (or porting) a number from one carrier to another initially took weeks, the FCC eventually mandated that the transfer take place within four business days.

Some cell phone carriers (such as Verizon Wireless) used this four-day window to attempt to convince customers not to switch. In response, the FCC in May 2009 changed the number portability requirement to one business day.

How to Initiate a Transfer

As of late 2009, the process has become very fast and painless. When you activate new service with a cell phone carrier, they’ll often ask if you’d like to transfer your existing number from another carrier. Transferring your phone number is free.

If they don’t ask and you do want your previous number ported over, make sure to let your new carrier know before you’re assigned a number there. If you request a phone number transfer, they’re required by law to grant it.

It’s very important not to cancel your current cell phone service until you have successfully transferred the old number to your new carrier. If you cancel at your previous carrier before establishing new service elsewhere, the number you’re trying to save will be lost.

To achieve a valid WLNP transfer, the cell phone carrier you’re switching to must offer local service in the same area as your existing phone number. Some carriers have online tools to immediately check your transfer eligibility (such as this AT&T tool).

Before You Transfer, Check Your Contract

While your previous cell phone carrier isn’t legally allowed to refuse a valid transfer request, you may still be bound to a service contract there.

If that’s the case, you’ll either have to wait until your contract expires or pay an early termination fee. If you’re with a prepaid wireless carrier without a contract or if you’re no longer under contract, you’re in the clear to initiate a transfer. "