Onto Flash now.  After a lot of searching and dead ends (And flash CS3 docs giving code for AS2!) i finally figured out an easy way to tween brightness of an object.  As this uses the Colour (Color) matrix transformations this also applied to colour tweening.

The Method: 

Perform a tween on a new object, and set it to tween an arbitrary property.  Set the easing and brightnes values (between 0 for dark and 255 for light), and timings as needed.  Then add a listener for TweenEvent.MOTION_CHANGE.  Within this event we build a brightness colour matrix using the current tween position and apply to the mc.filters array!

The Code:

import fl.transitions.Tween;
import fl.transitions.TweenEvent;
import fl.transitions.easing.*;
import flash.filters.ColorMatrixFilter;

var tw:Tween = new Tween(new Object(),"x",None.easeNone,200,0,5,true);
tw.addEventListener(TweenEvent.MOTION_CHANGE, function(e:TweenEvent):void
{
    var val:Number = e.position;
    var brightness_array:Array = [1, 0, 0, 0, val,
            0, 1, 0, 0, val,
            0, 0, 1, 0, val,
            0, 0, 0, 1, 0];
    mc.filters = [new ColorMatrixFilter(brightness_array)];
 });

I hope this helps someone out there!

Cheers,
James