D+D(press the key twice when you are in command mode) to delete the selected cell.
ESC to enter command mode then press Z to recover the deleted cells
Z(press the key to Undo a delete)
Shift-d: delete cell(press twice)
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In the new IPython cells could have two states: when it has a green selection rectangle around it you can edit what's inside; when it has a grey rectangle around it you edit the cell itself (copy/paste/delete). Enter/Return makes it go green, Esc makes it go grey. When it is gray, 'dd' will delete it