It certainly is. I remember seeing an article about this where someone was fitted with glasses which inverted the image. After a while the brain gets used to it and it seems normal. The subject in this case found that when he removed the glasses after a few days everything was then upside down as his brain got used to it and inverted the image for him.
Originally Posted by JBrunner
A similar thing happens if you watch TV whilst lying on the sofa. Although the TV image is actually rotated 90 degrees to your eye, your brain does the post processing and you think you are viewing it the right way round.