While I am convinced that technology combined with excellent instruction can allow the creation of new and powerful learning experiences for students, I am also convinced that "learning by computer" alone will never adequately prepare students for the complex challenges they will face as adults. Here's why...
Reason #1 - Learning is socially situated, and computer-mediated social interactions are a narrow approximation of human interaction
James Greeno (Stanford University) and numerous education researchers agree that learning is socially situated. The behaviorists of the 1920's were only partly right, it turns out. For example, learning to read a book is an important skill (behavior). But while reading a book by yourself engages your mind with the author's mind, it's only a one way street. Reading a book with others, and discussing it, then (in some cases) putting it to practice, brings the learning to new levels.
This is why I'm currently underwhelmed by current attempts to replace textbooks with electronic textbooks. While there is an obvious cost saving potential from eliminating the printing costs (and with open resources, the cost of content itself) and the backpack weight reductions are much appreciated, the exciting breakthrough will be when the electronic book does MORE than a printed book.
At Carnegie Mellon, Professor Ananda Gunawardena has shown what is possible through his "adaptive book" research. This electronic textbook system allows for the annotation of electronic text, but more importantly, the annotations are can be SHARED. Imagine margin notes that can be traded among study partners, and the ability download "expert commentaries" that can be revealed while reading. In essence, the textbook moves from a "solo learning experience" to becoming the foundation for shared conversation - and a new form of assessment.