Have you ever noticed that the iPhone doesn't seem to pick up all of the available Wi-Fi hotspots? That's because it doesn't. As another testament to Apple's focus on user experience, iOS doesn't display Wi-Fi hotspots with sub-par signal strength.
While this prevents the user from having a negative web experience, it also handicaps the device when searching for a web connection. How do you make your iPhone show all of the available Wi-Fi networks? There's a jailbreak tweak for that...