My money's still on PAX for the announcement. It's logical, captain.
Since Oblivion Bethesda has mostly relied on magazine announcements, although sometimes closely followed by a convention showing. This certainly doesn't exclude something new and now that Bethesda has a stronger presence in Europe and Japan they might be looking for something new. Bethesda went with Game Informer because it was multi-platform and had the largest subscription base in North America - a magazine preview also allows them to give a single source an in depth look at the game. Announcing a game at a convention lets it get to many more people but the information is more watered down. At the Oblivion announcement demonstrated it's also convenient to have a single source reporting on the game so you can tackle any misinformation with the second preview - if dozens of media outlets are publishing stories on your game unveil it's hard to follow up on that.
I'd put my money on a New Vegas style unveiling with multiple magazine previews. I also think it will be after New Vegas's release which unfortunately means it won't be at PAX. I think early 2011 is most likely but it wouldn't surprise me if they waited until the spring since this lets them announce the game and just as the initial furor is subsiding they enter Con season and start showing it to crowds.
Personally... I know if ESV was announced today, I'd probably go out and preorder FONV and play it several times over to get ready for ESV. Right now, I'll buy it, but I don't see the rush.
I think you're right. I can understand the reasoning that an Elder Scrolls V announcement would overshadow New Vegas' release but no one is going to forget about New Vegas just because TES V is announce. If anything I think it would boost sales.
However if Bethesda announced TES V now it would mean juggling a
lot of balls all at once. Getting New Vegas on to the shelf means they have that much more breathing room to throw TES V in to the mix.
In the end I bet both of these are minor considerations though. Bethesda is going to figure out approximately when they want TES V to be released and then figure out approximately how much time beforehand they want to announce it.