This week I received some transparent cubes from Korea.
With these cubes you can see the colour of some stickers on the non visible three sides. This means these cubes violate regulation 3h of WCA regulations 2006 v2.
3h) No modifications are allowed that enhance the basic concept of a puzzle. Some examples of enhancing the basic concept are: new moves are possible, normal moves are impossible, more colours/pieces are visible, moves are done automatically, more or other solved states.
Therefore these specific transparent cubes cannot be used in official WCA competitions.