The corner div overlaps Markers and catches their click events. (in Chromium 17 - Firefox 10 is fine)

Test case:
Use for example http://leaflet.cloudmade.com/index.html and move the map so that the Marker appears between the left edge and the zoombox. Now the Markers Popup won't open.

Chromium seems to have a bug in the z-index handling, maybe it has something to do with -webkit-transform.

In case of the zoombox this bug is not nice but doesn't hurt that much.
But if you put a bigger control on top of the attributionControl things get nasty.