Couple of things worth clarifying first (please correct me if I'm wrong)
1. Let's distinguish between a GM release and a public release. Even if they are the same build, they are released at different times, in a different manner, to different people.
2. Point updates (like x.1) never seem to get GM releases. They go from beta to public release.
I am pretty sure you are right that you've never been able to update OTA from a beta to a GM. Looking at
this article and its comments, it was certainly not possible with iOS 8.0.
That said, if my memory serves me correctly, the 7.1 public release was the first time you could update OTA from a beta to public release, and quite possibly this was also the case for the 8.x point updates too. Note again that I'm talking about beta to public not beta to GM, as there is no GM for point updates.
Finally, it could well have been possible to update from an iOS 8.0 beta to the iOS 8.0
public release, but I can't find any information on this. Most people would have manually updated to the GM by then I guess.
edit: Regarding that last point, I found
someone saying they updated from iOS 8.0 beta 5 to the public release. Note how a few posts above I expressed an interest in whether it was possible, hehe.
So, I think if last year is anything to go by, if you're running the latest iOS 8 beta then you won't be able to OTA to the GM when it's released to devs on the 9th, but you will be able to OTA to that build when it's released to the public a week or two later.