Update: We've got some new technical details about TDK's transparent panels.
TDK announced that they began to mass produce transparent PMOLED displays. The first display (code: UEL476) offers 2.4", QVGA (240x320) resolution and 40% transmittance. Other technical details: 150 cd/ m2 and a temperature range of -20 to +85 degrees C. TDK says that this display is not easily viewable from the rear - for enhanced security.
This is not very surprising - as we already know that Lenovo's S-800 uses a transparent PMOLED from TDK. So this is just a confirmation that the company indeed moved into mass production.
This panel uses color filters over a white OLED. TDK says that they are working towards more transparent OLEDs (probably in different sizes) - and also towards making more precise and longer lifetime panels, too.
TDK has been showing transparent PMOLED prototypes back in October 2010. Check out this nice video showing these displays (the display on show here is 3.5" in size):