From this day on (17 Juçu 2012) Lisbon’s airport have an underground station. You can take the subway and go to Sandanha Station, near center o of portuguese capital, in less than 15 minutes.
The new subway stations of the red line – Moscavide, Encarnação and Aeroporto – are open for service. The Oriente/Aeroporto line has now 3326 meters with a estimate mensal value of new passengers around 400 thousand passengers/year
In Lisbon’s Airport you can now go to the center of the city by taxi, bus, tube/subway/undergound, rented car or even by foot (not the best option).
Check all the info about the Metropolitano de Lisboa in the following institutional page.
Check lisbon’s underground map here.