Denethor Is Played By...
Cirdan

I get this informations today straight from New Line:

Twenty-sixth and last but one of the Ruling Stewards of Gondor, father of Boromir and Faramir, will be played by John Noble!

Denethor led Gondor in the War of the Ring, but the power of Sauron broke his will and he died by his own hand.

John Noble was Artistic Director of Stage Company of South Australia 1977 - 87. Since 1997 he is Head of Drama, Brent St. School of Arts in Sydney. Noble directed over 80 plays, including 'Sons of Cain' on London's West End (1986). He is a Freelance actor with semi-regular parts in TV series 'All Saints'. And he is a voice and acting teacher.

Movies:
Virtual Nightmare (2000)
Airtight (1999) (TV)
Nostradamus Kid, The (1993)
Sting in the Tail, A (1989)
Dreaming, The (1988)
Call Me Mr. Brown (1986)

Link to the IMDB.