Craig,

Thanks for the reply, it's working now

However, if you turn on the paging behaviour, your solution isn't working completely since the row_index on a new page starts at 0 again resulting in not being able to find the span tag (since the "index" field from the codebehind file isn't restarting). I fixed this by calculating the id of the span tag by myself by doing something like this:

var behaviors = grid.get_behaviors();
var selection = behaviors.get_selection();
var paging = behaviors.get_paging();

var rowIndex = selectedRow.get_index();

if (paging)
{
     rowIndex += (paging.get_pageSize() * paging.get_pageIndex());
}

var Id = $get("b" + rowIndex).innerHTML;