Source: Asked to me by Vivek Jha (Senior Undergraduate, EE, IITB)
Problem: Among 10 given coins, some may be real and some may be fake. All real coins weigh the same. All fake coins weigh the same, but have a different weight than real coins. Can you prove or disprove that all ten coins weigh the same in three weighings on a balance scale?
BTW, List of previously asked Coin related puzzles on the blog:
Coin Weighing Problem
Another Coin Problem
Five Thieves and Bounty
Update (Oct 12, 2010)
Solution posted by Gaurav Sinha (chera) (CSE IITK 1996 Graduate, Now working at Indian Revenue Service) in comments! Reposted by me removing typo.