Are there any substantiated health risks to a high protein, low-carb diet?
Too much protein is excreted so there are no health risks in that respect. You will also avoid muscle wasting with a high protein diet, however you're likely to have low energy levels from the lack of carbs so maintaining protein-only for more than one day isn't a good idea.
High-protein diets only pose a serious health risk if you do not take in enough fat and/or carbohydrates with them. The maximum for a high-protein diet is 60% protein. The human body only requires 0.36g protein per pound of body weight daily, and my understanding is that high-protein diets are usually regarded in the medical community as unnecessary.
Carbohydrates and/or fats are absolutely essential to both maintaining energy levels and maintaining healthy brain activity. Increasing protein sufficiently that you crowd out your energy calories is going to pose some serious side effects. Other than that, you run some risk of dehydration, and should watch your hydration levels carefully and drink pure water any time you're thirsty.
As always, before starting a diet like this, you should talk to your physician and make sure that your health history won't negatively impact your attempts at it. High-protein diets are meant for bulking up, not losing weight. Be sure to take supplemental vitamins, as you are not going to get all you need from a diet of lean meats.