@hungry donner if we're lucky we may see a gameplay trailer at e3? I'm pretty sure bgs is going to e3 with more than screenshots. After all they had a lot of footage for oblivion at e3 and this years e3 will be five months before release.
When Bethesda first showed Oblivion at E3 it was restricted to private demonstrations - if you weren't there you didn't get to see it and while the press wrote quite a lot about the demonstration that's different than released game footage. The showcase was eventually filmed and released to the public but this was months later.
In general Bethesda hasn't used E3 for really big announcements, they've focused more on playable demos and private demonstrations. Now the first FO3 gameplay footage was shown during a Microsoft E3 event, the game having been only recently unveiled, and I certainly wouldn't be surprised to see something similar happen with Skyrim . . . but I also wouldn't be surprised if we didn't get gameplay footage either.
I think Mikedzines is correct that Bethesda wouldn't want Skyrim to overshadow id's presence at QuakeCon. However I do think they can have several major releases at the con without stepping on eachothers' toes. August seems rather later for the first publicly released gameplay footage, perhaps E3 will give us a taste and QuakeCon will follow up with something more substantial.