To put it into perspective, ultrasn0w uses 152 bytes of properly malloc’d baseband RAM, which is 0.015% of what purplesn0w uses.  Put another way, purplesn0w uses 6900 times more RAM than ultrasn0w (and doesn’t let the O/S know that it’s using it, so the O/S still thinks it’s free to use.  When it does use it, the baseband will crash).

Also, there is another benefit. This uses mobile substrate to patch CommCenter unlike Geohot’s method which actually patches the CommCenter file. The benefit of this is that you are not modifying your system files and it is very easy to uninstall safely. You could probably just disable it with SBSettings mobile substrate menu as well.

I consider the current ultrasn0w the recommended unlock.

Dev team has released Ultrasn0w, the iPhone 3g unlock. Here are the instructions and info from the dev team site.

Ultrasn0w for iPhone 3G is ready!

1. Ensure you have upgraded to iPhone OS 3.0
2. Jailbreak your iPhone 3G using redsn0w or PwnageTool (this will also install Cydia/Icy)
3. Run Cydia or Icy
4. Please add the repo repo666.ultrasn0w.com to Cydia or icy
5. Search for ‘ultrasn0w’ in cydia or icy and install ultrasn0w
6. Reboot your iPhone 3G
7. T-Mobile USA users should disable 3G before using ultrasn0w
8. Enjoy

No, this will not currently unlock 3gs as that cannot be jailbroken yet.

I have spent some time researching options for all of us on 3g iPhones with a 2.30 baseband that cannot be downgraded. Yellowsn0w is not working for these devices, and if Fuzzy cannot downgrade your baseband, you are stuck with using a sim adapter. Unfortunately, not all sim adapters are the same. In fact, many are unstable and drop signal regularly. I have done some testing on various sim adapters and am pleased to report that the Gevey3g sim works very well and hasn’t dropped my TMobile signal once yet.

The sim does not require any sim card cutting, seems to offer full signal strength, and seems stable. It requires you enable data roaming if you want to get data (EDGE is all that’s offered here on TMobile USA).

In the process of my testing, I contacted Gevey3g and managed to get a small shipment of these sim adapters. They sell for $49.99 + shipping from gevey3g.com, but I’m offering them for a substantial discount to anyone that needs them (less than half price). Mind you, I am not trying to make a business of selling these and am only doing this to help out, because sim adapters are otherwise hit and miss. If you can find these for cheaper elsewhere, by all means feel free to buy them from someone else. I’m only offering a products that I personally tested and can put some level of confidence into selling. Note, I considered not selling these as I have prided my site around giving everything away for free. Therefore, I may stop selling these after the first lot is gone. There are also counterfeit versions of Gevey3g sim (since they seem to be stable) so make sure you are getting the real thing.

If you are interested from buying from me, you can use the google checkout link I have provided. All prices include shipping.

Before yellowsn0w, I used the “Virgin-Sim” sim proxy, and it worked quite well with T-mobile USA. When yellowsn0w was released on New Years Eve, I disregarded reading any forums and continued to restore my iphone to 2.2 (I was at 2.1) and install yellowsn0w. To my disappointment, it was a pain to get working (multiple reboots, airplane mode toggling, and sim removal and reinsertion).

yellowsn0w version 0.9.6 has seemed to solved most of the problems others and myself have experienced. No longer will you have to remove or insert the sim or toggle airplane mode to get signal. The Dev-Team is still working on issues with Sim Pins and such, but it cannot be long before they crack that problem. Unlocking the iPhone 3G can’t be any easier:

1. Restore your iPhone to version 2.2 (Not needed if you are already at 2.2)
2. Use Quickpwn to jailbreak your iPhone
3. Load up Cydia and add the yellowsn0w repo: Cydia source: http://apt9.yellowsn0w.com or Installer repo: http://i.yellowsn0w.com
   
4. Install yellowsn0w
5. Disable 3G and Data Roaming in Settings -> General -> Network (Step is mainly for T-Mobile USA whose 3G bands are not compatible with the iPhone 3G’s)
6. Reboot the phone and you should have signal. I have tested yellowsn0w on my 4 year old T-Mobile Sim and a half year old sim, both work perfectly.
7. If you have T-zones, you can get unlimited data by entering the APN: epc.tmobile.com in Settings -> General -> Networking -> APN

I was going to write up a guide and summary for the Sim-proxies\turbo sims that did work with the iPhone 3G, but now that is not necessary with a soft unlock. Remember, when a new iPhone 3G firmware comes out, do not upgrade right away. Wait until pwnage from the Dev-Team to disable the baseband update, or a version of yellowsn0w that does work with the new version of the baseband.

Kudos to the Dev-Team, thank you for this Christmas\New Years gift!

From the dev team blog:

3G Unlock

We have been working hard on a few other things. The main one being the 3G unlock codenamed “yellowsn0w”. This is now completed and is currently being packaged into a user-friendly application with the simplicity that you see in QuickPwn or BootNeuter.

• The target release date for the unlock is New Year’s Eve 2008.
• This unlock method is available to iPhone 3Gs that have 2.11.07 (FW 2.0 – 2.1) baseband or earlier, we did warn you.
• You can tell what version baseband you have by going to Settings->General->About->Modem Firmware
• The unlock requires a jailbroken 3G iPhone.  It’ll be installable via Cydia and so it doesn’t matter if you have a Mac or PC.
• Please refrain from updating your baseband, regardless of what version you’re at.  We’ll have complete directions on New Year’s Eve.
• We’ll stream a live demo of the unlock before Christmas (see the update at the end of this post)