Un-question that's probably been asked before: for gameplay purposes, does the acorn holostamp count as a silver border for cards like Border Guardian?

It counts as silver-border card for cards that care about Un-cards (like Ass Whuppin’). Border Guardian is the one exception because it cares about literal border color not Un-ness, so it’s the one that doesn’t count an acorn as “silver bordered”.

I assume this was answered with the first UNF preview, but do Acorn stamped cards count a silver bordered for cards like Ass Whuppin' or Border Guardian? Does it vary based on the card asking?

We’re going to errata most of the cards that reference “silver border” to say “silver border or acorn symbol”.

for silver-border cards that reference specifically silver-border cards/permanents (ass whuppin', for instance) are they going to be erratta'd to include acorn cards in the future? (or, will the term "silver-border" be considered equivalent to "acorn" in the future?)

Most cards referencing silver border will be errataed to refer up silver border or acorn cards. A few, like Border Guardian that specifically cares about border color, will remain the same.

Will cards like Underdome still work with acorn cards if R&D’s Secret Lair is out?

The plan is yes. With an exception or two (such as Border Guardian), “silver border” will become “silver border or acorn stamp”.

With the new Acorn stamp, will there be errata for cards like Ass Whuppin' and Border Guardian to count the Acorn cards as silver-bordered?

Ass Whuppin’, yes, as it is supposed to affect Un-games. Border Guardian whole flavor is that it actually looks at the border of the card, so that probably won’t change.

Will cards like Underdome that reference silver borders receive errata to work with "acorn cards"?

Basically, yes. A few cards like Border Guardian which is more about the state of the border rather than something being an Un-card might not get errata.

I need a silver boarder ruling please!! With border guardian in play if you play another unstable card since the bottom boarder is black does it get +1+1 and unblockable?

No, it’s a silver bordered card. All current cards have the black bottom and it’s not treated as a border for determining border color.

Does border guardian sit there unimpressed by your fancy invocation hazoret the fervent you just cast because it doesn't have a border?

No border is not a silver, black or white border.

So Border Guardian does nothing with the old gold-bordered World Championship decks?

Correct. It only cares about silver, black and white borders.