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At last after a very long time searching, I’ve found a keyboard shortcut that allows you to lock a Mac in a similar style to (⌃⌥Del, L) on Windows.
In iChat options set `Show status in menu bar`, and in Keychain also set `Show status in menu bar`.
Now when you press (⌃F8) the iChat status in your menu bar will get the focus. Then you can just press (←) once to highlight the Keychain status, (↓) to highlight `Lock Screen` and (↩) to select it.
This gives you (⌃F8, ←, ↓, ↩) as a rather convoluted and excessively long keyboard shortcut for locking your computer. It is however very handy when you are working in an office, need to go make coffee and want to [save on kittens](http://www.google.co.uk/search?q=%22every+time+you+touch+the+mouse+god+kills+a+kitten%22).