I don't particularly understand this casual use of the word "bubble." It describes every asset class or fad that comes along where the prices or beliefs exceed the facts and fundamentals. In fact, bubble in many cases is more dramatic term for fad or theme. Are we in a Twilight bubble? Is this a gold bubble? What about a healthcare legislation bubble?
Most every investment theme has its day in the sun. They are not all bubbles, or unrealistic, but there are always people that get more excited about an investment than is warranted.
Stop calling them bubbles, and realize that all trends come to an end. Trends have always been with us, and your job as an investor is to try and ride them to profits.