It's going to be a bit more than that, I think. The casting of Orlando Bloom and numerous other LOTR-only characters, coupled with the confirmed casting of Holm (which I only just found out about) indicates to me the whole film is going to be a flashback, probably set within the events of LOTR during one of the Rivendell visits; probably a later one as there's no sign of Arwen an Aragorn.
Pretty dumb, but hey they have to find some way to justify padding the thing to two movies.
Every other LOTR character that has been mentioned thus far (Legolas and Galadriel, specifically) can all very easily appear in The Hobbit
without it being a flashback like you describe. Elrond was already in
the book anyway, when Gandalf brought the group to Rivendell to translate Thorin's map. Also, as Emh
posted earlier, it's now confirmed that the White Council and their assault on Dol Guldur will play an expanded role in The Hobbit
. Gandalf, Elrond, and Galadriel (along with Saruman and Radagast) were all members of the White Council.
As for Legolas, well, that's easy. Legolas' father was Thranduil, who was king of the Elves in Mirkwood. That makes it very easy to place Legolas in some of the Mirkwood scenes when Bilbo, Thorin, et al
, are there.
Heck, if they really wanted, they could even have Arwen make a brief cameo during the group's stop in Rivendell. Aragorn could also technically appear, but at that time he was just a boy of ten years old who had yet to be told of his ancestry.