Has anyone actually "seen" entanglement? I want to know if the following has been done experimentally; after the spin (or any other characteristic with a probability of 50%) of 2 entangled particles has been measured, we change the spin of one and we see the spin of the other changing instantaneously at a distance. 
For example, entangled particle A is spinning up and entangled particle B is spinning down, we make A spin down and see B start spinning up at the same time.
I know entanglement has been proven experimentally but it always seems to imply this "spooky action at a distance" and I wonder if THAT has actually been proven experimentally. Maybe my question should have been has anyone seen spooky action at a distance...
 A: Do you mean literally see it?  I guess they are working on experiments like that:  http://www.wired.com/wiredscience/2010/06/human-quantum-entanglement-detector/
Or do you mean it is it physically real?  This says it is:  http://www.nature.com/news/quantum-theorem-shakes-foundations-1.9392
A: No. What you describe is not what is meant by quantum entanglement.  What you describe would allow instantaneous communication across large distances which would allow violations of causality and would violate special relativity.
Quantum entanglement occurs when you prepare two particles such that one is spin up and the other is spin down, but you don't know which is which.  You let these two particles travel (at less than or equal to the speed of light) to two measuring apparatus that can each be set at any angle, not just up/down, and measure the spins. What you find, when you compare both measurements is that the results are correlated as expected by quantum theory.  For example, if both apparatus measure at 45 degrees to the prepared spin axis, the measured spins will always still be equal and opposite.  See this for a full explanation.
There is no communication at all between the particles once they are separated. Thus, entanglement cannot be used for faster than light communication since it is only the comparison of results that is predicted.
Update:  Another great explanation of entanglement is here.
