The problem
I'm building a niche social networking site and want to implement a "Virtual Currency" much like this one on Stack Overflow, except I want to enable users to both earn currency based on activity and also to pay money to top up their accounts, if they want the benefits of a higher balance.
Existing implementations
There is an existing product I could integrate with http://www.srpoints.com/ which could be an option instead of building it myself from scratch.
http://www.mahalo.com/ also has a virtual currency. ( I will add more examples as I find them or you suggest them )
What guidance can this community offer when approaching an implementation of this "pattern"?