If I look straight up or down in a 180 degree the mouse sort of locks the x-axis so that I can't turn around.
But if I move down the mouse or up the problem's gone. Not a big problem, who needs sto stare 180 degree anyway, but still...

try doing the same thing in the real world - your neck will lock your head in the very same way.

Lol, so it's an realistic feature? Well okay then :)

I wouldn't think that's it. voodoo47's response doesn't seem well thought out as by his logic, a person can turn their head 360 degrees left or right without issue when looking at more typical viewing angles. A person can just as well replicate looking straight up or down and turning by simply moving their whole body and not just their head. I personally feel it's more of a bug or oversight than a "feature".

In either case, it is a rare circumstance and not a big issue as you've mentioned.