From the CR:
410.3 Playing a card or placing a resource adds that card to the chain. Using a power, completing a quest, striking with a weapon, or proposing a combat (collectively called “playing an effect”) all add an effect to the chain.

Therefore, placing a resource can be responded to, before the resource enters play.

Correct?

Also:

412.1a A player can place a resource only during his or her non-combat action phase while he or she
has priority and the chain is empty. Placing a resource follows the applicable rules for adding
links (707) and puts a resource card on the chain. Resource cards on the chain can’t be
targeted for interruption (710.1e). As a resource card resolves, it enters play in its controller’s
resource row.



NOTE: I could swear that at one time this wasn't the case, but clearly it is now. So if someone places a resource, it seems clear to me you can respond before that resource enters play.

I believe you're right. The reason this is confusing (I think) is because in Magic, placing a resource does NOT use the chain. Many MANY of the WoW rules are based on Magic rules, so it's only natural for people that have played both games (that haven't read the comprehensive rules) to assume most things work like Magic.

This is indeed correct.

I am almost sure this has changed since HoA.

I distinctly remember being told that you could not respond to placing a resource. So it looks like this must have changed.