Heidi's in the church twittering about her hair ... cameramen are rehearsing the church walk-ins and outs ... and a bunch of cops must be wondering, "Who did I piss off to pull this duty?"
Yep, it's Heidi Montag and Spencer Pratt's wedding day -- or as they say over at MTV -- a really big episode of "The Hills."
The ceremony is set to begin at 5pm on the West Coast, if all goes as scripted, but there's already plenty of action outside the Pasadena, Calif church.
We don't want to give away everything, but there will be a "scene" with Spencer tossing a bouquet.
By the way, one off-duty Pasadena cop who's working security in the neighborhood said this about the happy couple: "I have no idea who this is, but I know my daughter will be freaking out when I tell her."