No. I upgraded to the 3GS from a 3G and gave it to the wifey to replace her 2G. The 2G went to her sister that didn't preorder and is waiting for delivery.
Long story short: 2G SIM went into 3G. 3G SIM went into HTC Tilt. 3GS activated on 2G account. New 3GS SIM went into 3G. Old 3G SIM went from Tilt into 3GS. Old 2G SIM in a cupholder in the car.
Any questions? 😛