10W Supply should be fine with iPhone 3GS
The differences is the current rating. I have two different supplies for the iPhone. The latest one is the the very small one that does not have the exchangeable AC connector and appears to be quite low power. The first iPhone had a 2.5 W supply that looks identical to the iPad supply but is a 1/4 of the capability. Since the voltage is the same in all cases (5v), they should all work fine, but the iPad will charge faster with its 10W supply and it will probably make no difference to the iPhone. As stated in this thread the fact that all supplies share the same connector means that Apple will not have overlooked the fact that you could plug any other supply into any other device with the same connector and would make sure that no damage would result. Note that earlier iPods, for example, used 12v supplies but the power came in on another pin on the same connector used today. Thus connecting a 12v supply to a current iPad etc. will not cause any damage, they will just not charge or power the device.