I'm greek too, and I am really looking forward to this movie. While it won't be historically accurate, you have to remember that the main thing that has stayed from this story is the fight against impossible odds. This is what everyone thinks about when they hear about Thermopylae, to most, it is a tale of inspiration (which is what Frank Miller's novel emphasises over historical accuracy).
Just because that tale of inspiration isn't that historically accurate, doesn't make it a bad movie, or even a worse movie than one that would be more accurate.
I can see that the historical part is very important to you, and that is completely fine, but you have to understand that to the vast majority of the movie watching world, those historical accuracies come in second to an engaging "facing impossible odds" type of story that Thermopylae has always been about.
Seeing your credentials, I can safely say that I don't know nearly as much about history as you, but still, I like to think of myself as pretty up to snuff on Thermopylae, and while I will probably see many things in the movie that I know to be false, it won't diminish me from watching a (hopefully) great movie based off of one of the greatest stands of all time.