This is quite an old post while 3GS not out yet. I guess no. Anyway, please check on your firmware version before you find the correct way to jailbreak it.

No problem on that:

To upgrade to 3.0 or 3.0.1, you will need redsn0w to jailbreak then ultrasn0w to unlock.
To upgrade to 3.1.2, you will need blackra1n to jailbreak and unlock.

I guess I have a post on how to upgrade 3.0 and 3.0.1, but I am a bit lazy to do the 3.1.2 since the blackra1n is so much straight forward.

You may check on Internet for these tools.

I want to upgrade mine to version 3.0 as above.
previously mine have been unlocked with yellowsn0w. Is there any problem with the upgrade?

The answer depence on which version you plan to upgrade to. Difference version have difference upgrade method.

Thank you.

At the moment, my Iphone 3g is on 2.2(5G77) am I possible to use your matter above to jailbreak mine?

Thanks for linking me to the webpage. After posting my comment I went to youtube to try and find out more about unlocking AND jailbreaking my Iphone. Here’s the link to the youtube page.

Important information:
1. Plug in your iPhone and update to 3.1 or 3.1.2
2. make sure your on 5.11.07 baseband!
3. follow all the intructions to the letter and you shouldn’t have any problem.
Good Luck!

You may refer to this article: http://blog.iphone-dev.org/post/232020146/baseband-reprieve

Basically you are now able to unlock you 3.1.2 using this: http://blackra1n.com

The article is like below:

iPhone 3G/3GS owners who found themselves stuck with version 05.11 of the baseband (either by accident or because they bought it that way) are now in luck! geohot was able to turn the already-public at+xemn crash into an injection vector, which can be used to inject his version of the unlock. The blacksn0w unlock is available for free via Cydia by adding the repository http://blackra1n.com in the Manage->Sources panel. Congratulations, geohot!

Those of you who are already unlocked at 3.1.2 because you kept your 04.26 baseband now have an extra cushion of comfort, and more choices: ultrasn0w, purplesn0w, and now blacksn0w (and of course the original yellowsn0w too if you’re still back at FW 2.x). Whether or not you choose to update your baseband solely to use the new unlock is a personal choice, but so far there are no advantages to doing so (and remember you can’t come back to 04.26 after you’ve gone to 05.11).

As with all the unlocks, it will probably very soon be re-sold through scam sites that charge you money for what is offered to the community for free. Please stay vigilant for these scam sites and steer your friends away from them.

Update: Some commenters are reporting a lingering problem with WiFi while using blacksn0w. Some are able to solve it with a single “Reset Network Settings” but others say they need to do that periodically. So far there seems to be no pattern to those affected or the best way to fix it.