Are there cases of popular atheists who used to publicly defend atheism (via debates, books, etc.) but eventually ended up converting to Christianity?

There is Francis S. Collins, M.D., PhD., who was the head of the Human Genome Project and who has described himself as an 'obnoxious atheist' in his academic years. He believed that all questions of life could be ultimately reduced to 'physics and chemistry'. It was only in later life, in his work as a doctor when he was faced with a broad spectrum of suffering, and in particular, one woman who told him it was her faith that supported her, that he turned to reading C.S. Lewis's Mere Christianity. It was this that convinced him that all his arguments against religion was that of 'a schoolboy.'

He says it was only when "he was hiking in the Cascade Mountains on a beautiful fall afternoon, I turned a corner and saw in front of me this frozen waterfall, a couple of hundred feet high. Actually, a waterfall that had three parts to it - also the symbolic three in one. At that moment, I felt my resistance leave me, and it was a great sense of relief. The next morning, in the dewy grass, in front of the cascades, I fell to my knees and accepted this truth - that God is God and that Christ is his son and that I am giving my life to this belief.'

Personally, although you have described a 'strong' atheist as someone as very smart. I don't think this matters much in either atheism or theism - there are other faculties of the mind. And in this, especially, they matter more.

In fact, in a book that I'm reading at the moment, Islam and the Destiny of Man by Gai Eaton, a French-Swiss who converted to Islam and who his school-teacher in sixth-form described 'you are the only truly universal sceptic I have ever known' relates an anecdote of his time at Charterhouse, Cambridge:

A few of us had lingered on, drinking coffee, after the evening meal in the Hall at King's College. The conversation had turned to religion. At the head of the table say an undergraduate who was universally admired for his brilliance, his wit and sophistication. Hoping to impress him and taking advantage of a brief silence, I said: 'No intelligent person believes in the God of religion!' He looked at me rather sadly, before answering, 'on the contrary, nowadays intelligent people are the only ones who do believe in God.'

He saw right through him, and Eaton says:

I would willingly have sunk out of sight under the table.

What turned Eaton towards religion was a book called The Root and the Flower by the writer L. H. Myers, described at the time as the only 'philosophical novelist England has produced'. He also befriended him. Myers had given his entire life to the pursuit of pleasure. Few women had been able to resist his combination of wealth, charm and good looks. Although fascinated by spirituality and mysticism, he adhered to no religion and obeyed no conventional moral law. Later in life, he tried to change himself and 'even to repent his past', but it was too late. Three years after Eaton had first corresponded with him, he committed suicide.

Although Eaton named his son after him, he realised that his 'wisdom' had only been in his head and that it had never penetrated his human substance.

Although, it's again about an atheist turning towards Islam - Islam after all, is not far from Christianity - especially when measured in the atheism-theism axis, you might find the book The Road to Mecca useful. It's the spiritual biography Mohammed Asad, originally Leopold Weiss and whose father had been Jewish). He lists his reason as the spiritual emptiness of Western Civilisation and its idolatry of the Market and the misery that it has brought to everyone he knew. In Christianity, it's known as worshipping Mammon.

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@Mike Borden: According to Gil Eaton, on the axis of atheism-theism, Islam and Christianity are closer to each other than either are to atheism. Moreover, he says that Islam is closer to Christisnity than the other way around as Islam accepts Christ as a Prophet whilst Christianity does not accept Muhammed as a Prophet. Hence the direction of my answer which explored a number of spiritual conversions from atheism to theism - and in particular, Christianity.

I don't, for example, discuss conversions to Hinduism or Buddhism.