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.

David Heinemeier Hansson at Short Logic:

Groupon has filed its S-1 and hopes to raise $750M in its initial public offering. Given they’re currently losing a staggering $117M per quarter, despite revenues of $644M, they’ll be burning through that cash almost as soon as it hits their account. At the moment, it’s costing them $1.43 to make $1, and it doesn’t look like it’s getting any cheaper. They’re already projected to make close to three billion dollars in revenues this year. If you can’t figure out how to make money on three billion in revenue, when exactly will the profit magic be found? Ten billion? Fifty billion? My wife shops Groupon a lot. I do not. Partly because I feel I don't need to spend money on most of the things they offer and I know if I simply ignore Groupon, I will save money. I feel the bottom is going to drop out from under Groupon in a few years and everyone in the tech media will raise their hands and run around shouting about how no one saw this coming, how could this happen, etc. For the record, I saw it coming. ( 1:00 PM June 3, 2011 - for future reference)

Groupon Is Effectivley Insolvent; Like A Ponzi Scheme

Groupon: Spending $1.43 to make $1.00