Rodney, yourself and Alisdair both have valid points. If I can expand on one of your comments:
>>"Mass balance has nothing to do with that unless, in the process of adding it, you have changed the stiffness of the structure."
I feel it would be more accurate to say "resonant frequency characteristic" instead of "stiffness".

This resonant frequency characteristic is a function of both stiffness AND inertial mass around the axis of flutter, ie. by adding mass to the object (mass balance or otherwise) it increases its inertial mass (resistance to twist), effectively lowering its resonant frequency. It shouldnt matter whether you lower or raise the resonant frequency as long as it is out of the range of the frequency of the excitation force being applied.

When applying this fix, it is important to locate the mass as far from the centre of twist as possible to be effective.

The Aileron and wing should also be looked at as a system rather than independently and they act upon and influence each other.

After witnessing Spoilers wing failure, I was facing my F90 maiden with some trepidation, so I made up some weights on the ends of dowels glued onto the ends of the wings facing forward and some facing backward from the ailerons as well. It wasnt pretty, but I got 2 flights out of it before the flutter set in on the third flight on a slight overspeeding. I acutally saw the wing rather than the aileron flutter this time. IMO the wing is under built. If it had been build stiffer using less rib spacing and cross bracing (as per the ailerons), flutter could have been designed out. In fact Im going to have to apply this mod retrospectively as all the wing ribs broke along the lightening holes. The weight addition should be minimal and the stiffness increase 'maximal'! F90 designer please take note!