It's called the Observer Effect and technology is emerging to reduce the friction in the reading experience. It can help you if you're puzzled, deal with you if you're stuck, or even eliminate extraneous material if you're skimming.
Wired author Elliot Van Buskirk identifies the iPad as the first possible test of this tech. He notes Apple is involved in software breakthroughs to accommodate reading, an era of Text 2.0.
He concludes the new technology, once released and experienced for some time, could reinvigorate reading.