Wow, my first answer was actually completely wrong about OS X—I actually discovered this today, but a large part of the improvements to automation that Apple made for 10.6 make it possible to accomplish this through the use of services.
Open Automator from the Applications folder and choose the Service template. Specify Service receives no input in any application at the top. Then define your workflow however you want, either by using the Get Specified Finder Items and Open Finder Items actions, the Run AppleScript action, or the Run Shell Script action, and save the workflow as "Mount My Encrypted Disk Image" or whatever. It'll be automatically added to the services menu.
To assign a keystroke to this service, go to System Preferences -> Keyboard -> Keyboard Shortcuts -> Services, and assign a keyboard shortcut to "Mount My Encrypted Disk Image," which should appear under General.