A few minutes ago Apple posted a new GM seed in the iOS Developer Center. The build shows that it's a GM seed for iOS 4.2.1, with build number 8C148.
Here's what we know: a reliable source informed us that Apple released this build internally last night, and it's aimed at fixing major Exchange and audio issues, also documented on Apple's developer forums. The release of a 4.2.1 GM seed ahead of the rumored iOS 4.2 release (remember, it should have been out last week but Apple released a second GM seed for iPad to address WiFi issues) is curious and, indeed, our source also tells us Apple might just release iOS 4.2.1 to the public next week -- thus skipping iOS 4.2 altogether. We don't know for sure yet.
The Exchange and audio issues seem to be pretty serious, and Apple apparently asked to get testing done on this new GM build by this weekend "no matter what".
Like I said, this is very interesting. iOS 4.2 isn't even out yet and we're already seeing a 4.2.1 GM seed. We'll keep you posted as we find out more.