Well, in the second preview, she mentions that her dad has 500 cops looking for him. I'd have a hard time thinking she'd be saying that as a joke to Peter, and not implying Spiderman.
It's possible they're picking up after he's already gotten his powers and started being Spidey. Maybe Gwen will find out over the course of the movie. I would love this, we don't need another origin.
I think it would be funny if it opens with him saying something like "They made a movie about me once. As for how I got the ability to do the things I do, they got that pretty much right, though I have no idea who that "Mary Jane" person was; must have been a movie edit. As for everything that came after that... they weren't even close."
I imagine it'll be a super quick origin, since the previews have shown:
1. Uncle Ben alive and well (and the former POTUS, no less!). 2. Peter building the webshooter.
If I had to guess, I'd say it'll do like Incredible Hulk did and show the accident during the opening, then pick up after. That is purely 100% speculation on my part, though.
I forgot that I bought transition lens. Was walking back from class one sunny day a few months back and went down into a metro station. Couldn't see for about 10 seconds, and then I remembered I had transitions.