Your in luck.
Yes and yes. However if you are thinking about getting an iPhone 3G right now... DON'T! The new one is to be previewed tomorrow! (March 17) So I would just wait.
If you really want it now for some reason...
Yes, the 2.1.1 firmware (latest) can be jailbroken. I recommend using QuickPwn to do this.
When you buy your iPhone, get around buying service for it. Then when you have your inactivated iPhone you can jailbreak it, and use it. (Tip: When it is inactivated I think it's already in DFU mode, so you can just skip that.)
Or if you want to, you may be able to take somebody's sim card and use it with the phone temporarily to "activate" it. This way you can bypass jailbreaking it.
Good luck!