It was open for certain apps like Safari and iTunes, but not OS X.
Prior to the Mac Dev program restructuring at $99 for everyone (back when you got hardware discounts), I'm pretty sure that at least some betas were available to non-developers, but I might be misremembering things.
According to this Ars article, the AppleSeed program for OSX has existed since at least 2011 for non-developers (and for other apps since 2003, apparently). It's a different program than the (paid/registered) developers betas, I think.
But I think you had to apply for an invitation, and hope to get lucky and get picked to join.
AppleSeed, seperate from the developer program, has traditionally been for Apple's employers and partners, giving them early access to software and hardware.
It's more or less an 'invite only' program where Apple will tell you (or your employer) is allowed access.
Gotcha. I knew about it for other apps but missed it on OS X. But then, I was in the Mac dev program by then so I wouldn't ever look for/need an invite.
As others have mentioned AppleSeed for non-devs has been around for years. I've been testing major OSX releases since the end of 2008 for Snow Leopard - friend of a friend at Apple was looking for people to test exchange integration, IIRC.
I'm not sure this will give access to 10.10 seeds though - for the last couple of releases we've gotten redemption codes to install them initial seed - it's a separate application, not a software update.
