Pokémon surprise announcement this Monday
The Pokémon Company's official Twitter and Facebook accounts have sent out a message announcing that there will be a surprise announcement at 8 a.m. PDT this Monday, Sept. 16, 2013. These accounts have only advised followers to "tune in", and have not specified the medium on which the announcement will be presented.
The teaser logo is similar to the logo shown at the end of the video that was accidentally uploaded to Pokémon's YouTube channel earlier this week, indicating that this announcement may be related to that video.
To check the announcement time in your timezone, see here.