The Wi-Fi hotspot feature is reported to be available on iPhone 4, not on iPhone 3GS. Current iOS 4.3 beta 3 supports up to 3 devices according to a source who insisted on anonymity.
Feb 19 21:04:25 unknown misd[29] <Info>: checking for carrier provisioning
Feb 19 21:04:25 unknown misd[29] <Notice>: carrier service is available
Feb 19 21:04:25 unknown misd[29] <Notice>: maximum number of wireless tethered hosts is 3
Feb 19 21:04:25 unknown misd[29] <Debug>: wrote changes: success
Feb 19 21:04:25 unknown misd[29] <Debug>: published changes: success
Feb 19 21:04:25 unknown Preferences[61] <Warning>: BTM: setting discoverable status enabled
Feb 19 21:04:25 unknown Preferences[61] <Warning>: BTM: setting device pairing enabled
Feb 19 21:04:25 unknown misd[29] <Info>: checking for carrier provisioning
Feb 19 21:04:25 unknown misd[29] <Info>: carrier service is available
Feb 19 21:04:25 unknown misd[29] <Info>: maximum number of wireless tethered hosts is 3