You have a couple of options, some of which have already been stated:
1) Hack the iPhone and use how you want, however this will in most cases void the warranty if you can't get the original DB reinstalled. With this method you can use the iPhone as an iTouch
2) You can use the iPod/Video functions without a hacked phone but you will have to activate it first then inactivate the account. It is the only way to get it into iTunes and use the Store.
3) You can wait...You say this summer is when your current account ends. I would recommend that you wait or buy an iPod touch for the interim because sometime this summer, probably around June or later, they will be releasing version 2 which should have alot more functionality and capabilies...we hope. Then get that phone and you will have the most recent to go with your new 2 year AT&T plan.
Hope this Helps...Good Luck!