Default solution
In IPython 2.x and 3.x (cmd|ctrl)-/ works but requires an english (american) keyboard layout, see https://github.com/ipython/ipython/pull/3673.
Other keyboard layouts
In case you have a non-english keyboard layout, you can define a custom keybinding for the codemirror editor via your custom.js. To this end add e.g. the following lines
define([
'base/js/namespace',
'base/js/events'
],
function(IPython, events) {
events.on("app_initialized.NotebookApp",
function () {
IPython.Cell.options_default.cm_config.extraKeys = {"Ctrl-," : "toggleComment"};
}
);
}
);
to use Ctrl+, to toggle (block) comments. I use this with a german keyboard layout and IPython 3.0.
The previous solution (see edits) worked fine with chrome, but not with firefox.
Old solution (IPython 1.x)
If you are using IPython 1.x you can try the comment-uncomment.js from https://github.com/ipython-contrib/IPython-notebook-extensions – I haven’t tried this yet, but I guess its a good start.