For anyone that got the 3G after having a 2G iPhone and wanted to erase all data on the 2G before selling it - would like you feedback. Here is what I think the process will look like:
1. Activate new 3GS
2. Sync it so it brings all my data/apps over
3. Erase all data from 2G using built in data wipe without needing to hook it up to iTunes
4. Sell old iPhone to buyer
I am pretty sure the new iPhone will come with a new SIM because I think the buyer needs a SIM in the 2G to activate or use it on his AT&T account.
As I was writing this, I found this
AppleInsider | Secure data wipe built into iPhone Software v2.0 which I had not known about. Seems pretty straight forward now - I originally thought I'd need to connect to iTunes to erase old iPhone which I'd be afraid would cause problems.