This week's book giveaway is in the Other Open Source APIs forum. We're giving away four copies of Storm Applied and have Sean Allen, Peter Pathirana & Matthew Jankowski on-line! See this thread for details.
Singleton as a design pattern only explains solution and its upto developer how he implements it as per the system architecture ( clustered environment etc). So, unless we are aware of your design ,cannot comment on how it can be cracked.