The package update which contains MS portion of 8.1 and the carrier approved firmware from Nokia may be rolled out as early as sometime later this month.
Carriers approve the entire package. They make no distinction between firmware and the OS. They contribute to neither, they only test and approve it.
I'm
waiting on the full att OS. The dev preview is a partial. It doesn't include any if the carrier portion.
Like I said, there is no such thing as a 'carrier portion'. I don't know where this misconception is coming from, but it's widespread apparently.
What carriers might provide are a few apps which are downloaded from the store during initial setup. Those apps aren't even shipped with the OS however, nor are they relevant in terms of stability.
For each of their customers (the carriers), OEMs will add unique configurational info to their firmware, but that you already have in the 8.0 firmware that is on your device. That won't change with the official update.