It is possible to assign a keyboard shortcut to any action on the PDF menu of the standard OS X Print dialog. Add a shortcut for Save PDF to DEVONthink Pro (or whatever the PDF-to-DEVONthink command is called on your system) by using System Preferences > Keyboard > Application Shortcuts > All Applications. In every application that uses the OS X Print > PDF menu, you’ll find your new shortcut.
In conjunction with the OS X-standard command-P shortcut to invoke printing, you can have a two-shortcut routine to quickly get a PDF into DEVONthink.
This works in Lion. It’s supposed to work in Leopard and Snow Leopard – I don’t have them so I don’t know. Lion is cranky - the shortcut works sometimes, sometimes you need to repeat a few times to get it to work, and sometimes it just never works. I’d say I have a 70% success getting Lion to recognize the custom shortcut. Simpler shortcut variants are more successful (e.g., command+some key). For extra credit - it might be possible to combine command-P with your custom shortcut using QuickKeys or Keyboard Maestro.