It appears (I am trying to figure out the checkbox too) that there is NO VALUE assigned to the checkboxes at all. They are ignored when passed.
But I am working on extending the following snippet which works at the JS level:
for ( var row_id in mygrid.checkedRows)
{ //check which rows have been Ticked
console.log('selected row: '+row_id+' '+mygrid.checkedRows[row_id]);
console.log('Row data: ' +mygrid.getCellValue('uid',row_id));
}
Where uid is the name of the column which denotes the Unique Identifier of that row. (the checkedRows array is 0-based)