What if I put SIM from my old iPhone into new 3G iPhone?

As stated in the title - do I need to do a whole new activation with a new 3G iPhone, or can I simply pop in my old iPhone's SIM and continue on my merry way?

I couldn't find any threads or articles discussing this aspect, sorry if it's a repost.

Well, I recently heard the news that you cannot purchase a 3G iPhone without activating it in the store. Which means you'll be stuck with 2 years of the $30/mo contract, making the question of using your old SIM a moot point. I wonder what the second-hand 3G iPhone market will be like?

Oh well, GPS integration was the big seller for me, but with the increased monthly cost in addition to the purchase price I'm questioning the value. (3G is nice but not a biggie for me personally.) I figure that iPhone 2.0 software on my old EDGE iPhone will give me plenty to play around with. 🙂