This is one of those life questions that comes up with fair amount of frequency so I've probably given it some semi-serious thought at some point in my life. I'm guessing that my answer then was likely what my answer is now and that is that whether I see the glass as half empty or half full depends upon what kind of day I happen to be having.
I believe that even the uppest of of optimists likely has a day or two that the glass is looking a little more like it's being steadily emptied and even us grouchy worry warts have days that all seems right with the world. What I try to do is look at the glass of water and see just that, a glass of water neither half full nor half empty but one that simply is. Which I suppose might be something similar to acceptance or letting go of attachment/expectation but it's far too early in the day to read that much into it and right now my cup of coffee is decidedly on the empty side and in need of refilling.