World War Two aircraft, at least combat aircraft, generally had oxygen systems which are needed above 15,000 feet (and helpful at lower altitudes). Since open flames and oxygen systems are a very bad combination, smoking in a World War two aircraft would have been extremely reckless.

Edit: some bombers had ashtrays apparently, (nice photos /u/aquanaut) so I guess they must have worked around the danger of the oxygen system somehow.

Adolf Galland had a cigar lighter in his 109. The following is from a HistoryNet interview with him

WWII: Is it true that you had the only cigar lighter­equipped Messerschmitt in the entire Luftwaffe?

Galland: I think so, plus a holder for it if I was on oxygen. It created quite a controversy, I can tell you.

[B-17s came equipped with ashtrays.] (http://www.warbirdinformationexchange.org/phpBB3/viewtopic.php?f=3&t=33341&start=75) B-29s too I believe.