Animated masks

I was given this logo and was told to do some kind of flash to it

[img=http://img366.imageshack.us/img366/6321/compuyata0zf.th.gif]

I was thinking the arrow could move around the outline of the logo so only what the arrow has moved over is shown through a mask.

I’m very rusty on mx 2004 (havent been designing in flash since v5)

I tried to motion tween a square mask but when the arrow goes around the curves it doesnt look right. I also tried shape tweening but that still didnt help.

Is there an easy way to do an animated mask like that with out having to go frame by frame?