I came across an interesting article in the Feb/Mar 2010 issue of the Australian Sky and Telescope magazine. It is titled "Where have all the aliens gone? by Jacob Haqq-Misra and Seth D Baum.
In 1950 Enrico Fermi "...made a back-of-the-envelope calculation to estimate the time required for a technological civilisation to spread throughout the galaxy."
"So if any technological extraterrestrial society exists, then they should have visited us by now."
Arguments put forward to explain why we are not being visited by ETs include:
* Life on Earth is a statistical anomaly
* Civilisations blow themselves up
* A Prime Directive may prevent them from revealing themselves.
The authors of this article question Fermi's assumptions and suggest that "If growth outstrips resources, human civilisation may collapse." Therefore, perhaps this explains the absence of ET visits to Earth.