I once made up an action point system for Shadowrun, which was pretty cool, but horribly complicated and it was a burden to track multiple NPCs.
An action point system for D&D would be pretty simple to do, since the actions are already defined in a similar way (just not as detailed).
The biggest problem - the one I had faced above - comes, when you actually want to make it different from the standard system, in other words, if you want to split actions so that it results in a common flow, where everyone acts, not the current one person acts, then second person acts, then third person acts, etc.
This is what I had done, based on the Champions initiative, where the combat round is split into several smaller segments and you get only fractional actions (measured in action points) during each segment.
One option to do this would be to get rid of rounds completely (you can count time for spells and such in another way) and simply start the combat at zero, then count up in action points spent as actions are being done.
For example, an attack costs 50 action points (but for ease of play the points are spend after resolving the attack), moving 1/10th of your base move costs 5 action points, etc. Now you simply add up your individually spent action points and have an "initiative count" move up from zero. Whenever the count comes to your individual current total, you can act.
Initiative can be handled with a delay, so that you start the round not at zero, but at some other total.
Well, there you have something to work with!
Bye
Thanee