And now, with mere hours to go, I'm offering up my final guess.
Something We've Never Seen Before
If you look at the most recent major British royal weddings, the precedent actually leans towards the bride not wearing some well-known vault gem.Diana married wearing her family's Spencer Tiara rather than the Cambridge Lover's Knot Tiara she received from the Queen. Sarah married in a newly purchased tiara, chosen by the bride and bought by the Queen from Garrard's. And Sophie married in a tiara composed of existing pieces from the royal vaults (thought to be perhaps elements from Queen Victoria's Regal circlet) but newly designed for her use.
The Middletons may not have a family tiara like the Spencers did, but that doesn't mean Mr. and Mrs. M. couldn't spring for something sparkly for their daughter. After all, they've got to be turning a tidy profit with all this additional publicity for their children's party business. Or, Charles: he has some pretty deep pockets himself, and he certainly seems to enjoy purchasing jewelry for Camilla.
So, let's place our final guesses. What do you actually think she'll wear? Or do you think that she'll break my heart and go tiara-less?
Oh, and P.S.: There will be no Flashback Friday post tomorrow. Something tells me we'll all be busy anyways.