Oops, just did...

I hate the red "1" sign on the top right, so I always upgrade everything as soon as it's available.

Just successfully recovered the old version in iTunes. Here is how I did it:

First of first, go to iTunes ASAP, locate your most updated iPhone backup and make a copy before you try any recover trick.

1. delete the new version of authenticator on your iPhone
2. disable auto sync in iTunes
3. Connect your iPhone via USB
4. Click your iPhone, then go to "application" tab
5. On the left side, scroll down, you gonna see the old version of authenticator, install it.

So close...