Should SF Be More Optimistic?

You should all be reading Futurismic by the way. It’s a wonderful blog with a lot of great articles, such as the one I’m about to talk about.
Mr. Raven recently wrote about SF being so dismal and depressing and whether or not it should try to be a little more optimistic. While I agree on principle, I think there is something being forgotten about SF. SF isn’t a genre about bubbly happiness and it never was. Yes, there will always be that level of “awe” where new and bizarre things take place that inspire and strike people with interest. But, SF has evolved from the Golden Age and it is primarily dealing with real world issues that we may one day face. The genre is like an early warning system in that way. It deals with realistic issues of what may be based on the technological advances of today (for the most part at least).
There is also the idea of conflict, which is central to any work of fiction being successful. Conflict, unless in a comedy, isn’t generally happy, since that would sort of defy the nature of conflict, barring of course the possibility that the main character loathes being happy.
Is optimistic SF possible? Of course it is, and when you look at SF it is very optimistic. Granted, it deals with issues that are negative (technology going wrong, people blowing themselves up, war, etc.), but generally the end result is optimistic, right? Maybe a way to look at SF is trying to see the overall picture: that despite the negative future that might exist, mankind will prevail.
But then we’re left with that negative future, which is the root of the issue, I suppose. Well, I think it would be incredibly difficult to move away from that inherently negative future. Part of what creates conflict in SF is that technology does go wrong, people do freak out and blow each other up, interstellar wars will exist so long as us humans exist. These sorts of things are just being realistic. There are always going to be negative things about society, no matter the time. If we look at today we can see all the negative bits in the world. Genocide still exists; governments are going wacky and doing crazy things; economies are struggling, etc. Perhaps being optimistic is to look at the bright side of things, and perhaps what makes SF so pessimistic is that the bright side is often only survival. To be optimistic might mean that SF needs to approach the future from a point where everything has gone right and only one person has done something stupid. I think of a murder mystery. Society is advanced, technology worked out, and wars mostly don’t exist (at least any more than they do today), and all you have is a cool detective hunting down a bad evil man who murdered and old lady. So, there’s optimisism, since the invention of technology has changed the world, in theory, for the better (maybe cops can do things better, crime is relatively gone in such a massive world that would be somewhat overpopulated, etc.). But there’s still the negative aspect to it, of course. I don’t know.
So, what I ask is this: is there a way to write optimistic SF without it still being somewhat pessimistic? Is having the main character or the good guys win enough to make it optimistic? What exactly is optimistic in SF? Is it just having technology do nothing but good, and if so, how do you create conflict from that?