I bet it could work with the 3GS but Apple is just lazy in doing that. They want to market it as an iPhone 4 only feature. The 3GS processor is rated at 833MHz but is downgraded to 600MHz, The iPhone 3G and 2G are capable of 600MHz but are downgraded to 412MHz. The iPhone 4 A4 processor is the same 1Ghz version from the iPad bit is probably downgraded as well to about 800-900MHz (we will know soon) because in tests it performs slower than the iPad. Apple most likely does this for better battery life but it kinda sucks your not getting the full potential of the product your buying.