Can I use a Verizon phone on T-Mobile?

Episode 1119 (1:58:10)

Steve from Glendora, CA
Apple iPhone 4

Steve was gifted an iPhone 4 and the carrier is verizon, while his carrier is T-Mobile. Leo says Steve can't use it because the iPhone 4 is a CDMA phone while T-Mobile uses GSM. So it's essentially apples and oranges. It's just not compatible. The new iPhones though are "world phones" and can use various SIMs. Leo says that's where it's going, especially for Verizon because they were ordered by a judge to stop locking their phones to a carrier.