If you want to pick up an extra iPhone 3gs for the kids, my suggestion is wait a couple of weeks, they are selling for a premium at the moment on eBay. Average selling price is well over $250.00 but in a few weeks demand is going to slow down and I am expecting you will be able to pick one up real cheap. This is a great way to get a nice phone for the kids or someone that is not sure if they want a smart phone.
At the price they are selling for now, it is better to wait because 1.5 million people of whom about a million of them will now have a iPhone 3gs laying around that they may want to sell. Thus I think the price will drop. With a new iPhone with iOS 4 selling for $299.00 with a 2 year contract the current selling price of a used 3gs at $250.00 is a little high.
If you have one you want to sell at a decent price you can hook a geek up. I am looking for 2 of them.. Email me with your offer! If they are jail broke even the better!