For those of you who own the Samsung Vibrant and have been experiencing issues with the Mobile AP app that comes as stock, it looks like it’s finally been fixed.
The Mobile AP feature lets you set your Android device as a wireless Access Point to connect to the internet on other network devices with WPA encryption.
XDA forum member Jr33 has posted that the app is now up and running on the Vibrant, thanks to member rhcp0112345. You can download the fixed app but it needs to be flashed with Clockworkmod recovery and does not work with an AOSP ROM.
Once installed, you can access Mobile AP via Settings>Wireless and network >Mobile AP Settings.
For more information, including the download link, take a look at the forum thread.
Update: This is a mod to add in the Samsung version of Mobile AP intended for the Galaxy S, but was removed by T-Mobile._________