I apply this attribute to a span and it still selectable.
---
There's a good chance that you are selecting another object first (like the body), then dragging the selection into the area occupied by the span. "An element with the UNSELECTABLE attribute set to on can be included in a selection that starts somewhere outside the element."
The unselectable attribute prevents the onselectstart event if you click on the element directly, but not if the selection begins elsewhere. For this reason, applying the attribute to a single span is much less effective than applying it to the entire document body, for example. (jsudds)