Note 2010-11-17: there's more than one way to do this. See the updated post from 2010-11-17.
Here's an example of some R code I posted a few weeks ago on making QQ plots of p-values using R base graphics.
The Perl highlighter also works well. Here's some code I posted recently to help clean up PLINK output:
Simply head over to gist.github.com and paste in your code, select a language for syntax highlighting, and hit "Create Public Gist." The embed button will give you a line of HTML that you can paste into your blog to embed the code directly.
Finally, if you're using Wordpress you can get the Github Gist plugin for Wordpress to get things done even faster. A big tip of the had to economist J.D. Long (blogger at Cerebral Mastication) for pointing this out to me.