Check out this announcement from MTR:
HK$1.5 discount for MTR passengers interchanging from five Outlying Island ferry routes
From 16 July 2011 to 15 January 2012, Adult Octopus cardholders taking the Outlying Island Ferry services from Sok Kwu Wan, Yung Shue Wan, Peng Chau, Cheung Chau and Mui Wo to Central to interchange for the MTR will be able to enjoy a $1.5 discount off their MTR fare. All they have to do is swipe their Octopus cards at the new Fare Discount Processor installed at the respective ferry piers before entering the MTR network through Central or Hong Kong Station on the same day.
The Fare Discount Processors are located at:
(1) Sok Kwu Wan Route : Before exit at Central Ferry Pier 4 (Sok Kwu Wan Route exit)
(2) Yung Shue Wan Route : Paid area of Yung Shue Wan Ferry Pier
(3) Peng Chau Route : Paid area of Peng Chau Ferry Pier
(4) Cheung Chau Route: Unpaid area of Cheung Chau Ferry Pier
(5) Mui Wo Route : Unpaid area of Mui Wo Ferry Pier
The Interchange Discount is not applicable to Airport Express and the free MTR connection journeys offered by Airport Express.
For details of the new Interchange Discount, passengers may refer to the MTR website at www.mtr.com.hk or call the MTR Hotline at 2881-8888 during office hours.