Half-full implies that half of something is missing, or that more expressively, something is said to be half way full. The emphasis is on the contents, i.e the degree of its 'fullness'.
Half-empty on the other hand implies that there is half of the contents remaining, thus emphasising what is missing.
To counter Hivetyrant: Although it may sound illogical, it all depends on how you read it. Half-empty does not imply (at least to me) that you have half of nothing, but rather that half of something is gone, and hence that the other half remains.
Conclusion:
Phenomenologically, both are valid descriptions. They imply the same idea but emphasise perceptual difference.
Half Full - What is there
Half Empty - What is missing
To push the significance of one's preferences to be a sign of optimism or pessimism I think is a bit absurd though. I'm notoriously pessimistic (to those who know me) yet find bi-polarity towards linguistic questions like this a little annoying. (To see what I mean, check out the bored v.s violent poll)
Heh, I guess it depends on how rational you want to be about threads like this too.
