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.
So if you have to specify the host platform, i guess they will use the same to test your program. i'm not testing it on another machine than my pc with winXP. i also don't have the money to buy me a unix/mac just for testing purposes