It is entirely possible that the teaser advert could be directly related to the flagship store (e.g. a new shopping experience or the availability of an already announced handset, such as the Asha 311).
With that said the September 7th date does tie in with the already announced Nokia World event (September 5-6th). Nokia World has been split into smaller events this year, returning to the pattern Nokia used in the early 2000's, but it remains a likely candidate for device launches.
With Windows Phone 8 due sometime in the second half of 2012, and the presence of a Windows Phone device in the advert, it is not unreasonable to speculate that the teaser could be pointing towards Nokia's first Windows Phone 8 device.
Image credit: My Nokia Blog
Reading the QR code displayed in the advert leads to a website (http://m.nokiagraffiti.me/) with a similar message and branding. There are a number of linked pages including "Graffiti Me" (create your own graffiti in the style of the campaign), "Show Me" (hinting at new things in the new Nokia store) and "Find Me" (instructions on how to find the Helsinki flagship store).
Let us know what you think in the comments.