according to Jeremy Clarkson it is possible to smoke in a plane toilet and not set off the smoke alarm by holding the flush lever continually and putting your head down the bowl.
Don't know if anyone has ever tried this though........
Aeroflot, Air India, CAAC (China Airlines)...and there are more. The best thing to do is when you're planning on travelling, just check with the carrier.
I will offer that if you're travelling within the EU, you will not find a carrier permitting smoking on board.