Folks had forgiven (or at least "not blamed") AT&T after noting that the iOS 4.3 page said only three wireless connections would be allowed to a mobile hotspot, but information given to BGR points to it being a carrier decision. The default is five, but if the carrier requests its, it can be lowered to three.
At this point, this would mean we can go back to blaming AT&T for this, and back to hoping the Verizon will allow 5 wireless connections to its mobile hotspot app when it "catches up."
At the same time, however, sources have said iOS 4.3 will launch earlier than expected. It was supposed to launch Friday with the iPad 2, but reports are it will become available to iDevice users through iTunes at 10AM PST. Hold onto your broadband connection, if that's true.
Update: it seems it won't arrive until tomorrow, at the earliest, but considering the iOS-frenzy, it's no wonder these rumors continue.