Accessing data in subgrid

I have a grid with subgrids set up and want to access the data of a subgrid when I catch an outside event. Something like this…

myGrid = myLayout.cells("a").attachGrid();
myGrid.setColTypes("sub_row_grid,ed,ed");
myGrid.attachEvent("onSubGridLoaded", function(mySubGrid){
	// setting up mySubGrid
});

myForm = myLayout.cells("b").attachForm();
myForm.attachEvent("onButtonClick", function(buttonID){
	if (buttonID == "DoStuffOnSubGrid") {
		myGrid.forEachRow(function(rowid){
			// Here I want to access data in the rows subgrid, but I can't find anything in the grid API that will let me do that. Please help
		});
	}
});

I’ve deliberately left out most of the setup for myGrid and myForm, including loading them from xml. Assume the form is loaded from a static XML file and the grid and subgrid are loaded from PHP-generated XML and that all of this works fine.

I mostly need to read cell values from the subrids, but being able to update single cells without destroying the entire grid structure and reloading from server would be sweet.

Any ideas?