While the legal issue is the focus here if I was Groupon I would focus on my employee retention. I get that a company is trying to take sales people but see if they are leaving, and why they are leaving rather than how to close the gate.

It's interesting to see how company's are using the growth of intellectual property rights to encroach individual freedoms to pursue (or be pursued) for better employment. Limiting someone's employment options is tantamount to slavery or indentured servitude at best--both illegal in the U.S., and no individual property rights should ever encroach a person's right to grow his/her careers. Forcing people to sign such nonsolicitation agreements is certainly unethical if not illegal.
A company certainly has a right to protect the "classified" information that employees possess as a result of working for a company. The military, government, and intelligence communities have been using classified information for years, and the obligation not to disclose it lasts a lifetime or until such information is declassified. Other than a flimsy contract that could easily be shown as coercive, I would be interested to see how Red Hat would try to litigate a suit like this without doing permanent damage to their recruiting & retention efforts. After all, none of Red Hat's options in this case would encourage its employees to stay. It would actually have the reverse effect. And beyond that, Red Hat would have the burden of proving why it has the right to ask for such concessions from its employees in return for the "privilege" of working for such a company. After all, losing an employee causes Red Hat no more harm than any other company who loses an employee, and that is the cost of doing business in a free society.
Using an example from history, Xerox was known for decades as having the best sales training program in the world, and their sales reps were pursued by anyone who wanted to move a product in the market. Many stayed with Xerox. Many left, but Xerox never stopped trying to have the best sales force in business, and they didn't try to encroach the freedom of their employees by stifling contracts to force them to stay. So what did they do to keep people? You'll have to research that for yourself.
The benefit of the "free agent" employment world that both sides of the political aisle like is that competition for talent raises wages and creates opportunity for new entrants to join an industry in growth. This is the magic of economic growth for the middle class in capitalism. Anything that limits an employee's ability to change jobs moves us toward slavery--clearly the least ideal "job" their is.
Shame on you, Red Hat. Your problem is clearly short-sightedness & lack of leadership, and no contract or crafty lawyer can fix that.

I suspect Groupon's impending failure has more to do with their busines model than their sales force, and that's why Top Hat is interesetd in them. What I find more interesting, is that a failing organization (if, indeed, they are failing, which I think is just supposition) would threaten to spend money to stop employee poaching. This would be an expensive legal action.