Screencapture Command and Multiple Monitors


I need to totally automate a screencapture command for each screen attached to a Mac Pro, running either Snow Leopard or Lion. This would happen hourly, saving each file to the Sites directory, for viewing online (e.g. MONITOR1 -> ~/Sites/monitor1.png, MONITOR2 -> ~/Sites/monitor2.png).

This is literally the task, so I can't unfortunately include human intervention/interaction in the process.

I know how to write the script to do most of this, except I don't know how to specify a monitor, or otherwise get an image of every attached screen.

Can the monitor by specified in the command? Can I tell it to capture all screens to separate files?