Acknowledgements
We thank the Journal of the Virtual Explorer editors for inviting this contribution, and Mike Brown and Linc Hollister for reviewing it. Financial support from Italian Ministry of Education, University and Research (grant PRIN 2007278a22 to BC), from a Ramón y Cajal research contract to AAV, and from Ministerio de Ciencia e Innovación of Spain (projects CTM2005-08071-C03-01, CSD2006-0041 and CGL2007-62992 to AAV).