Thanks to DarkUFO reader Bigmouth for the following. I'm pretty sure this is true but until we can get official confirmation we'll place in the rumours section for now.
I have it on good authority that Patrick Fischler of AMC's critically acclaimed Mad Men will be playing a character in episode 5x08, which is currently filming in Hawaii.
Fischler is an extremely talented actor whose work on Mad Men was recently praised in a New York Times Magazine story about the show. My source was uncertain what role Fischler would play on Lost, but said source did share one tantalizing detail. Apparently, Fischler was in the late running for the role of Daniel Faraday, which eventually went to Jeremy Davies.
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Source: Bigmouth@Eye M Sick