Went ahead and assigned @paulirish on this.

Chrome/Safari as far as i know support copy/paste/cut events and allows to use event.clipboardData.setData;
IE supports window.clipboardData ... and fires copy/paste/cut events (although, seems, this events is fired only when some text is selected)
Opera - http://www.opera.com/docs/specs/presto2.10/#m210-294
Firefox - seems, support copy event, but not allows to use "clipboardData" - https://bugzilla.mozilla.org/show_bug.cgi?id=407983

There is a proposal for a clipboard API http://www.w3.org/TR/clipboard-apis/

the question is: "how to detect, that event.clipboardData.setData or window.clipboardData.setData is supported?"

Opera 12.50 supports clipboardData

@Yaffle do you have a proposal on the detect? i dont know this area too well

I think, document.queryCommandSupported('copy'); should work fine these days.
For Fifefox < 41 it returns true so it should be checked that document.queryCommandEnabled('copy') != undefined does not throw an error;
And document.queryCommandSupported is undefined on Opera Mini...