According to Macrumors.com (iPhone 3G Launch Day, Meetups and Activation - Mac Rumors):
This would be ideal. The packaging is intact and the process is faster.
Reports of required in-store activation in the U.S. remain, but one unconfirmed tidbit we've heard is that customers with existing iPhones will be shunted to a separate line and will simply need provide the new IMEI number (printed on the outside of the box) from their iPhone 3G. Activation will reportedly then be completed at home after switching your sim card to the new phone and plugging into iTunes. If true, this would speed up sales for those upgrading their existing phones and also leave the iPhone 3G's packaging intact (to the relief of many).
This would be ideal. The packaging is intact and the process is faster.