I had a 3GS and when I ordered the iPhone 4, I added a new line. However, I wanted to use the iPhone 4 as my new phone (it was already activated on the new line)
Planning in advance, I did the following:
1) Ordered micro-sim card from AT&T online. The only one they have available to purchase is for the iPad...but it is just a standard micro-sim card. They state it is for the IPad, but like I said there is no difference between it and the micro-sim for the iPhone 4. As long as it hasn't been activated on an iPad, you are fine.
2) Got a replacement "standard" sim from my local AT&T store for the 3GS
After I got my iPhone 4, I contacted AT&T. After they verified the phone number I wanted to use the new micro-sim with, they had me read off the numbers on the front of the micro-sim card and within 2 minutes it was ready.
I placed it in the iPhone 4 and it was ready to go with my original/old number. We just repeated the process for the 3GS. I gave them the number off the front of the "standard" sim card and my old iPhone 3GS was activated with the new line's number.
So now, the old 3GS with the new number will go to my mom and I get the iPhone 4 with my old number.
Hope the above makes sense, and is helpful!!