What type of roast has the lowest caffeine content? I'm aiming to reduce my caffeine intake, but don't want to drop coffee altogether. What roasts / beans release the least amount of caffeine into my body when drank?
In general, darker, arabica roasts have less caffeine. Arabica (as opposed to robusta) inherently has less caffeine in it naturally, and darker coffees have less caffeine since they have been roasted longer or at higher temperatures, essentially "burning" the caffeine out of the bean.


EDIT

For some science, Arabica is usually found to be about 1.2% caffeine and Robusta is typically 2.2% caffeine


  Source: Differences between Arabica and Robusta Coffee


Chris in AK has a great explanation in the comments as to why darker roasts are perceived to have less caffeine


  It is a common misconception that darker roasts have less caffeine.  Caffeine's melting point is 455F which is well above what most beans reach during the roasting process.  Essentially, caffeine content changes because the mass/density of the beans change (they lose water and undergo changes in the roasting process).   If you measure your coffee by volume, dense greener beans will have more caffeine.  If you measure by weight, darker roasted beans will have more caffeine.