Layer --> Transform --> Fit to Comp and hold Ctrl down while moving to enable snapping. Otehrwise numerically dial in the position.
thanks a lot Mylenium!
Also you can set a top layer to screen or multiply to align things that may be in the images that are the same shape. For example if you have several shots of a table with a bunch of things on the table that were shot hand held and you want to line up the table top, set the blend mode of the top image to screen align the table top visually, view the composition at a 200% or better zoom factor, and nudge the shots into position, then reset the blend mode to normal. Another blend mode handy for lining up similar images is Difference. I use these blend modes all the time to line up elements in a scene. Check these before and after images.
This trick can be very helpful when doing things like hiding a microphone or light stand that crept into a shot.