"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
iPhone 3GS Has More Powerful PowerVR SGX 535 GPU?
SGX520 (7 MPolys/s, 250Mpx/s) for the handheld mobile market
SGX530/1 (14 MPolys/s) for the handheld mobile market
SGX535 and SGX540 (28 MPolys/s) for handheld high end mobile, portable, MID, UMPC, consumer, and automotive devices
SGX540 (1000M pix/s, 20-35M Polys/s), SGX545, SGX555
iPhone developers, however, have discovered that the iPhone 3GS has extension files named "IMGSGX535GLDriver" suggesting that the new iPhone uses the more powerful graphics processor intended for "high end" mobile devices. This may not be entirely conclusive evidence by itself but it is consistent with a report from a Anandtech commenter who claims to have heard directly from Apple engineers at WWDC that the iPhone 3GS does indeed use the SGX 535. As seen from the above list, the 535 seems to deliver much greater performance over the originally believed 520 model as well as the 530 model found in the Palm Pre.