I've found out awhile ago that Apple was still signing off firmware 4.1 for my 3GS , I've used SHSHit app to test this a few times now for the last 3 weeks and Apple is still signing it off..so if i'am right if you get a iPhone on 4.3.4/4.3.5 there is a very good chance that you could downgrade your device to 4.1 and have a untethered jailbreak..and you will be able to save those SHSH's
Another report says the Apple is still signing iOS 4.2.1 for the iPhone 3GS.
We have not had a chance to verify these claims; however, if true, those who are stuck on a tethered jailbreak should immediately save their SHSH blobs giving you the ability to downgrade. You don't need to be on a specific firmware to save SHSH blobs for it.
Instructions to save your SHSH blobs can be found here: (Mac, Windows)