Apple has released iOS 4.3 to the general public so you can now update your iOS device.
The update, among other things, will enable you to share your iPhone 4's Internet connectivity with other devices using a feature called Personal Hotspot.
And contrary to the copy on Apple's iOS page, three seems to be the maximum number of devices that can simultaneously connect to an iPhone 4 running Personal Hotspot. I connected my iPhone to my MacBook Air and shared the Internet connection via USB and then successfully connected two other devices via WiFi. However, I was unable to connect another device to my iPhone via WiFi.
It's not a big deal. I don't anticipate wanting to connect more than three devices at any one time anyway. But it's still odd that the Apple page says that Personal Hotspot will connect up to five devices when the limit seems to be three.