The cell phone may work from Miami, but not sure about when you get into the Caribbean, sorry. My daughter used hers once on board Explorer in New York this past week-end and it was Ok from there. As for the different prices of the cabins, a lot depends on where the cabin is located even thought they are the same size. Same goes for outside, balcony, etc. I would think being the first "official" voyage you may probibaly get a few extra suprises. I'm sure it will be very exciting, enjoy it and let us know how you like the ship. I hear its just beautiful with tons of neat artwork throughout :-)
I can't speak specifically to your cell phone or service but I will tell you that Motorola tri-mode StarTac on Verizon works with no problem and is handled as a simple per minute roam just as anywhere outside home area. This is both in San Juan and St.Thomas.