Here are a set of extremely nice tutorials from Hadley Wickham (creator of ggplot) and whom we interviewed here at http://decisionstats.wordpress.com/2010/01/12/interview-hadley-wickham-r-project-data-visualization-guru/
Hadley is teaching two short courses at Vanderbilt university (read at http://gettinggeneticsdone.blogspot.com/2010/07/hadley-wickhams-ggplot2-data.html)
or download code and presentations here at http://had.co.nz/vanderbilt-vis/
They are a set of very very lucid and easy to understand presentations so even if you know very little R, or just want to learn visualization you can refer to them.
Here is the first lesson by Prof Hadley on Basics in Visualization using R.