There are definitely significant differences with the two phone for the opposing carriers.  As soon as you open the Verizon iPhone 4 a few things are quite evident.

Things that are different:

• vibrator
• speaker housing
• cable protector
• cable goes under the battery to reach the logic board
• logic boards have different connectors in differnt places
• More screws holding the board down
• Headphone jack
• Different cover for the front facing camera
• Dock connector flex
• Different frame for the glass

Things that are the same:

• Battery
• Screen