I've found this bottle of Nuka Cola.
Guy says that this is the original promotional Nuka Cola and wants a lot of money for it.
http://img709.imageshack.us/img709/5519/nukai.jpg

I think that the original Nuka Cola looks different:
http://media.giantbomb.com/uploads/2/28922/1023998-nuka_super.jpg

How do you think? True or fake ?

According to the Fallout Wikia, it is indeed a fake, the Nuka Cola bottle from the promotional events look like http://images2.wikia.nocookie.net/__cb20080906091015/fallout/images/7/75/Real_Nuka-Cola_Bottle.jpg.

So no, I would not buy it for collecting 'authentic' promotional items.

Edit: I'd like to clarify, the reason it's fake is the neck of the real bottle is long and thin, the one you were considering is short and thick.

Hmm ... I've found on the flickr another strange picture:

http://www.flickr.com/photos/escaped_monkey/697518673/

Looks like fake Nuka Cola above ?
Second fake because I don't think it's the same bottle ?

It's hard to say, but I'd expect Bethesda to point out whether it's fake or real.

If I were you, I'd PM Gstaff or Jason Bergman and ask if that is the real deal or fake.