Given the common situation of responding to a context menu launched after a long press on a list view its probable that you want to know which item was long pressed.
I found this obscure and took a little digging to find, when in the onContextItemSelected its easy to work out with item in the context menu was selected, what I could not find was which item was currently selected in the list that was long pressed to get the context menu.
The answer is:
AdapterContextMenuInfo menuInfo = (AdapterContextMenuInfo) item.getMenuInfo();
This object contains information about the position of the selected item in the list view.
Another bit of advice is that when extending the BaseAdapter class is to return a real data object form the getItem() member rather than the int as a position (as in the example I followed when I did the first one) The reason is that other wise you will need to keep a separate copy of the items in the list to use when the user selects an item to get the details.