A number of iPhone 3GS owners have reported slow and inconsistent data speeds, along with widely varying amounts of signal strength, when using Wi-Fi.
Users note that signal strength can fluctuate between one and full bars even when standing right next to the router or when a computer nearby shows a constant signal. This goes along with a commensurate reduction in speed. Even at full signal on both the computer and the 3GS, one user reports getting a maximum of 3Mbit downloads on the iPhone while getting nearly 7Mbit downloads on a MacBook using the O2 network. The fluctuating signal strength problem has affected me since I brought home my new iPhone 3GS.
The other, more widely reported problem is a sizable speed decrease in the iPhone 3GS as compared to the iPod touch and iPhone 3G tested under the same conditions. In one case an iPod touch garnered download speeds of 4579ms and uploads of 1363ms, while the iPhone 3GS showed only 1360ms down and 160ms up.