I sent mine in but they did nothing at the problem. They sent it back within two days claiming there is no problem and that they had updated firmware (which they could not have done since I did it before I sent it in lol...)
I called support again and they in return will call the repairshop asking what they actually did to my tablet. Most likely (if everything turns out ok) I will have to send it in again (which is still is annoying) and hopefully they will notice the problem this time.
Not to defend Sony in any way, but it can be difficult to spot the problem when the screen brightness is anything below 50%.
I notice my problems in the following scenarios.
1) when I boot up the tablet, first the Sony logo. Here you can see it a bit because the background is black. But it gets a lot worse when moving into the next screen during 'Xperia'. Here the brightness is turned up, making it extremely noticeable.
2) when I am trying to watch a movie outside or in a bright place making my screen reflect, forcing me to turn the brightness up above 70% mark...
3) when I am using my tablet in pitch dark... Even in low brightness setting it can be noticed. Especially bad when the movie scene I am watching is for example night time and thus naturally contains a lot of black on the screen.
Anyway, thanks for your info. Before I submit my tablet, I will make sure the recipient sees and understand the problem and get that person to attach a note to describe it for the repair crew.
Judging by the lack of response to this thread, we seem to have encountered a rare problem.