Jailbreak semi-tethered your iphone using sn0wbreeze 2.9.3

Well,
for those who want to update there iphone 3g/3gs baseband to 6.15.00 using iPad baseband, then use sn0wbreeze 2.9.3 on windows.

you can create the custom firmware with latest iOS 5.1

for that you need (download links are given with the names)
1- iOS 5.1

Here are the official download links to iOS 5.1 directly from Apple:

• iPhone 4S
• iPhone 4 (GSM)
• iPhone 4 (CDMA)
• iPhone 3GS
• iPad 3 WiFi
• iPad 3 GSM
• iPad 3 CDMA
• iPad 2 WiFi
• iPad 2 GSM
• iPad 2 CDMA
• iPad
• iPod touch 3G
• iPod touch 4G
• Apple TV 2G (5.0)
• Apple TV 3G (5.0)

2-iPad firmware
3-Sn0wbreeze for windows
4-windows pc

Procedure is simple

1- download the softwares
2- open sn0wbreeze
3-go to expert mode and when prompted click yes on upgrading to iPad baseband,
4- build custom firmware
5- then put your device in DFU mode using sn0wbreeze or Redsn0w
6-then open iTunes and restore it with that.

voila.

done..

you have updated your iphone 3g or 3gs to iPad baseband with iOS 5.1

Now you can use fuzzy band and downgrade the baseband to the ones unlockable or
you can try ultrasn0w to unlock it.