Could a real-life X-Wing fly in Earth's atmosphere? From an aerodynamic point of view, could a full-size aircraft of X-Wing design fly in Earth's atmosphere? Assuming you were free to add control surfaces here and there, could the wings in open configuration create lift ?

 A: You've seen biplanes.
Almost anything can fly if it has enough area, an angle of attack, and is nose-heavy.
I've seen a model airplane in the shape of Snoopy's doghouse!
It flew just fine.
If you're wondering why nose-heavy it's this.
Look at a normal plane with a main wing and a tail. It's nose heavy.
The main wing holds the weight by lifting up.
The tail keeps the nose from dropping by lifting down.
So there's a torque moment between the wing and tail that is balanced by the weight of the nose, and it is proportional to speed squared, just as lift is.
Now if the plane slows down, that torque moment decreases, causing the nose to drop.
When the nose drops, the plane starts going downhill, picking up speed.
That causes the moment to increase, causing the nose to rise.
So that nose-heaviness, and the moment, stabilize the plane's speed.
If the plane is not nose-heavy, it doesn't seek a natural speed and is very hard to control in level flight. If it is nose-light, it can even nose up and then slide backward.
The way normal planes control their speed is by a trim wheel that creates a little more or a little less downward lift on the tail. Then if they want to climb they use more throttle, or to descend they use less.
So it's counterintuitive, but the throttle does not control speed, except on a very short time scale. It controls the rate of ascent/descent. The trim wheel controls speed.
Some planes don't have a tail but instead have canards on the nose.
It works on the same principle. The canards hold up the nose.
So higher speed, nose goes up, plane slows down, nose drops, etc.
Planes with just a delta wing also have a moment.
It's just built into the single wing.
EDIT: Bernhard wanted a picture. Here's one:

A: There are a few concepts for jet biplanes that have been developed over the years, although I'm not aware of a high performance jet biplane that has actually flown. 
You can view one of the concepts here at the bottom of the page:
http://media.aerosociety.com/aerospace-insight/2011/03/08/asian-aerospace-2011-day-1-highlights/3853/
And another one here:
http://web.archive.org/web/20000929172415/www.lmasc.com/ama/tanker.htm
There do seem some other fundamental issues with the design such as the centre of gravity vs. centre of lift and straight wings on what I assume is a supersonic plane but given it was developed somewhere far away I'm sure that these could be fixed. 
A: Vehicles in the star wars universe make use of an anti-gravitation drive in order to levitate or maneuver within a gravitational field.
It should be able to fly but, from outside appearances, it does not appear capable of a controlled, unpowered descent.
