What you think about the fences around graveyards, Richard?
Yet another kind of threshold - one that only effects ghosts. Maybe spirits but I'm betting only ghosts.
When Harry met a Black Court Vampire beside his grave neither one of them was worried about an invitation. Harry's powers were at full (unlike when he entered an apartment uninvited in White Night) and the Vampire didn't need an invitation to enter. If that had been a "people live here" threshold then things would have been vastly different at that meeting.
No, now that I think about it, it doesn't affect spirits. In Grave Peril a spirit that looked like Harry took someone to a graveyard. Magic was tossed around. A Fae wandered in. No signs that anyone needed an invite.
More recently (GS Spoiler)
Jim has hinted that Demonreach projected itself into the cemetery and was the extremely powerful thing that had problems communicating with Harry.
which wouldn't have worked with a threshold that effects spirits.
My take is that we are talking about different types of "thresholds". I might be wrong about this, but this is how I see them broken down:
"Home" thresholds that prevent nasties from entering, degrade spells that cross them, and strip away powers from wizards who enter; can be invited through one.
"Dawn/sundown" thresholds that destroy vulnerable spirits and degrade spells but has no effect on wizards; no invitation attached to this.
"Graveyard" thresholds that keep ghosts either in or out; no invitation (other than an opening in the wall) applies.
"Holy ground" thresholds that keep nasties out but have never seemed to effect Harry's magic (even when he was brought in unconscious); invitation is debatable.
"Run water" thresholds that keep nasties out, degrade spells that cross them, but a wizard wading through a stream doesn't leave his power behind; no invitation applies.
"Magic circle" threshold that keep nasties out (or in) and are broken if someone with freewill passes through them or throws something through them, or if the circle is damaged either by someone with freewill or the environment.
Then there's:
"Created boundaries" like runes carved into things that can be used to anchor a ward.
"Natural boundaries" that can be used to anchor and define a veil.
Here's hoping that I've clarified my views on this topic.
Richard