Well, not a good news. The upcoming untethered iOS 5.1.x jailbreak developed by Pod2g will not work on iPhone 3GS and iPod Touch 3G.
This is a little weird because earlier Pod2g said that it will work on all devices. Even today he posted iOS 5.1.x untethered jailbreak FAQs and confirmed that its compatible with all devices. May be that was hypothetical and now he is actually testing it on every device.
Next devices that he is going to test are iPad 2 and iPhone 4S. Hope it goes well. Other than that he has already confirmed his exploits working on iPhone 4 and the new iPad (iPad 3).
Anyways, those of you who own an iPhone 3GS old bootrom have nothing to worry because their devices are untethered for life. New bootrom iPhone 3GS and iPod Touch 3G have to rely on tethered jailbreak until he or someone else comes with a better solution.
We will keep you updated on the upcoming untethered jailbreak by Pod2g. Stay tuned.