Another good writeup I found that expounds on what David Heinemeier Hansson said earlier this week. Conor Sen, writing for Minyanville:

So a company that owes $230 million more than it has, and appears to be burning through $100 million or more a quarter, is using money raised from later investors to pay back early investors? Sounds vaguely familiar. I'm not accusing Groupon of doing anything illegal or unethical. Ponzi, Enron, and Madoff all swindled their investors by misleading them about the financial health of their enterprises. As Minyanville's Todd Harrison likes to say, "The only difference between intervention and manipulation is communication." Groupon is telling you exactly what they are in their filing forms and by their actions. Invest at your own risk.