I ordered Halo 2 on ebay (UK ebay) last Monday, and it arrived yesterday.
Spent quite a bit of time playing it last night, and am very impressed. Running at 720p widescreen, the game's actually quite pretty. Obviously the famed Halo storyline and gameplay are exactly the same, but the gfx are much better than I was expecting for a last gen title.
If anyone with a 360 hasn't played Halo 2, I'd recommend you get it and get familiar with the Halo universe before Halo 3 comes out.
I paid £12.00 including postage for a brand new copy of the game.