It's true; they're mentioned in an article in the New Yorker on digitization of information:

Six hundred years later, Eusebius, a historian and bishop of the coastal city of Caesarea, in Palestine, assembled Christian writings in the local library. He also devised a system of cross-references, known as “canon tables,” that enabled readers to find parallel passages in the four Gospels—a system that the scholar James O’Donnell recently described as the world’s first set of hot links. A deft impresario, Eusebius mobilized a team of secretaries and scribes to produce Bibles featuring his new study aid; in the three-thirties, the emperor Constantine placed an order with Eusebius for fifty parchment codex Bibles for the churches of his new city, Constantinople. Throughout the Middle Ages, the great monastic libraries engaged in the twin projects of accumulating large holdings and, in their scriptoria, making and disseminating copies of key texts.

(h/t to Bill; thanks!)

If you've ever wondered what the weird Roman numeral/Arabic numeral stuff is in the inner margins of the print NA27 ... well, now you know. For more information, check out Kevin P. Edgecomb's page on the Eusebian Canons.