This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
It's called variable interpolation, and the standard Properties mechanism doesn't support it. There are however third-party libraries that do. I'm pretty sure Apache Commons Configuration does, but you should have a look at the documentation to be sure.
Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.