As always I recommend using computer-aided study to master this skill quickly:
PC / MacRobert Whelan has a piano keyboard drill up at emusictheory.com, which will run in any java-enabled browser.
iPhone & iPad
If you've already sprung for Tenuto ($4) you have two nice keyboard quizzes. This app is highly recommended. If, for some reason, you want an app that focuses exclusively on piano keys you could try Play Piano HD ($2) for iPad, Piano Notes Fun ($2) for iPhone, or Piano Notes Pro ($3) for iPad.