I would say with a script as the only possible way the script would do several operations in sequence to accomplish the final effect but fro the user's perspective it would be one task..
Depending on version and whether there is artwork outside the Artboard, which has to be dealt with if there is artwork that is sucked inside, you may do it in a limited number of steps, which would also be the steps in a script.
With no artwork outside, you may be able to create a nostroke nofill rectangle corresponding to, or replacing, the Artboard in one step (in 10, you can Object>Crop Marks>Make with nothing selected and then Object>Crop Marks>Release), select all, scale to fit the new Artboard size, and with the Artboard Tool Crop to Artwork or something; once you have the Artboard rectangle you may skip that part (in later scalings).
With artwork outside, you may be able to use the Scale Tool to both select an arbitrary point of origin and scale proportionally by a specific factor (I believe this is possible after 10), you may select all, set the Artboard origin as the point of origin, and scale, unless the Artboard origin moves when the Artboard size is changed.
Well you cannot do it without a script however if you want to do something manual because of proportional concerns you draw a rectangle the same dimensions and location as the existing artboard.
Then either group the art and artboard or simply select both of them and scale to the percentage or using the selection tool resize them by dragging.
Then you ungroup and or select the rectangle only and go to Object>Convert to Artboard.
I woud have suggested this but cannot see how this can be made into an action that would have any time saving or workflow value.
You would still have to draw the rectangle manually and then select the rectangle then run an action including a scale step and the an ungroup but first I think you would have to stop the action to select only the rectangle,quite a messy deal.
I think a script might work better.
It would be different if there were a command to covert artboard to a path and if that were the case an action would work but without this command there is no real way.
However if you use CS 4 and like the old filter create crop marks you can load the last version of it into the plug in folders for Illustrator CS 4 and it will work.
what is the required artboard size. I am used to putting a H x W dimension in, not just one number