Some SWFs use LZMA compression, see: http://swf.codeazur.com.br/#3269bcb435191bc4ceefdd664fee79d1a33b31d89b08fea1c38acab16132f9e1
LZMA can be used by any Flash SWF since version 11; not directly related to Flex.
(In reply to Aaron Kaluszka from comment #2) > LZMA can be used by any Flash SWF since version 11; not directly related to > Flex. Is it just available in IDE publish settings or widely used by flash developers, games and ads networks?
I would guess primarily in publish settings.
By that I mean that I do not know the percentage of published SWFs using LZMA, though I don't think it's insignificant at least for games. However, using the LZMA functions manually outside of publishing seems less likely.