I have a 3gs, untethered, 3.1.
I want to install 3.1.2 so that I can jailbreak it, but when I shift+restore, and choose 3.1.2, which I downloaded, it tells me that after contacting Apple, I can't do it. So, I'm guessing that what he said is true, and only 3.1.3 is available now.
In other words, I can't upgrade to 3.1.2, since 3.1.3 is out. And there is no way for me to downgrade from 3.1.3, especially without a SHSH file, because I've never jailbroken my phone before.
I'm just going to stick to 3.1, and see what happens in the next couple of months.