When precomping a bit of animation for a larger comp, eg creating a logo rotation etc, when you bring it in and want it to loop:

rick-click Time>enable Time remapping

then alt/opt click the stop watch to enter the expression
loopOut("cycle");

slide the coloured layer handle as long as you want it to loop and bob's yer da.

have been melting my head on ae expressions all night trying to get something that will "move to the beat"

Math.sin(time*7);

is about right it seems, and then you mulitply that by what ever you need the unilt to be, ie if you want a solid to flash to the beat, stick that expresion on its opacity with an extra times 100 on the end:

Math.sin(time*7)*100;

Thanks to Mr Gifford and Mr Ebbert for their helpful sites...