Getting an iPhone would RENEW your contract for 2 years. Not sure if it will leave you on the same voice plan, but you do have to pony up $20 for the unlimited data plan.
I have an iPhone if you have any other questions.
Oh yeah, you can add international calling and international data for a monthly fee if you travel a lot. If you need to, I believe you can call and add those, and then have them removed if you're not going to travel abroad. I have read one guy's review of using his iPhone in Germany and a couple other countries.... and he had issues...