This is a discussion topic for the Ore project, ToolSwitch. View the full project on Ore for downloads and more information.
ToolSwitch
This plugin will automatically switch tools for you as you begin breaking a block. Demonstration here
Commands: /toolswitchtoggle, tstoggle - toggles whether the auto tool switching will activate for the player using the command.
Permissions: toolswitch.command.toolswitchtoggle - permits use of the toolswitchtoggle command