I wonder if it means anything that some anonymous soldier is narrating the trailer.
Before some EA clerk accidentally revealed that it is indeed ME3, the whole trailer was designed to trick the viewer that it's about something else, up until the final shot showing the reapers. Which is why the character isn't Shepard and he's not wearing N7-like armor (we've also managed to convince Bioware to drop the visor they wanted to include), or why you don't see modern buildings or vehicles, or why we've desaturated the rifle's colored parts. So I don't think that it has any other meaning.
(Yeah, we did this trailer, and I've been jumping up and down when I've learned about it
It also seems like you'll be playing while the invasion is occurring. If so, that means the Reapers have reached the galaxy a lot quicker than I originally thought.
As far as I know time is advancing in the ME universe between releases, too. So when ME3 starts it'll be about two years after the destruction of the collector base.
As for how they got back, well, since they've built the mass relay network, there must be a way for them to cross great distances without system, too. They should be able to accelerate to incredible speeds, after all even a derelict reaper had the energy to maintain position in a planet's gravity field for 37 million years.
As for the story, I'd be disappointed if it would be as simple as building a fleet and fighting a last great battle. In fact, I'd be disappointed if there would be only one ending and I couldn't decide to make piece with the reapers, or have humanity replace them.
And let's not forget, we still don't know a lot of pretty important details, like where the reapers are from, why they're doing this, and what's going on with suns and dark matter. If anyone has read the Gateway novels from Pohl, there's one possible source of inspiration, and Clarke's 2001 seems to be another one - knowing the Bioware writers, I'd expect some surprises and a far more divergent story depending on the player's previous choices.