I didnt have this exact problem but before i bought my iPhone 3G i was planning to use the charger i got with my 4G iPod (i think the last iPod with a black and white screen) with my iPhone 3G as that is the one i use with everything else.
When i got my iPhone 3G it didnt work, not sure if it recognised it (ie said this product is not compatible with iPhone 3G ) or just did nothing, but it didnt work, so i had to use the bundled iPhone 3G charger.
Another problem i had along these lines is that the car charger i used with my touch 1G didnt work with my iPhone 3G (it said this product is not compatible with iPhone 3G), so i had to buy a Griffin PowerJolt SE for my iPhone in my car.