NO! Apple is not signing 3.00 or 3.01 firmware for the 3GS.
The only exemption is if you have your Keys hashed at Cydia on their server, the window was only narrow
The 3GS iPhone needs any firmware to be signed. Hence why custom firmware won't work on the 3GS.
I have a feeling that jailbreak will only be possible on 3GS with 3.1 as there is no way to sign a custom 3.1 upgrade for a 3GS with the old baseband.
That is why there is still no 3.1 jailbreak for the 3GS. It's going to be a full apple 3.1 with new baseband or stay on 3.00 / 3.0. Until the signing algo can be broken - if ever!