Are there going to be unlockable classic designs? New colour schemes I could see, but whole new models (unless they're simplified, which would be fair enough) seems like a lot of work for an easter egg. Mind you, it would pull in punters who would otherwise be put off so it might well happen.
As to the screencap... the background is definitely in-game rendering. The character model is difficult to call, but after a friend told me the jaw-dropping cinematic Halo 3 trailer was all done in-game, I reckon it's not unrealistic to assume that this shot is in-game too.