Is there a keyboard shortcut to jump directly to a particular full screen app in Lion?

If you want this for a specific app, you could use an application like Keyboard Maestro to set up a shortcut to switch to that application.

If you want this for every app, I don't think it is possible at the moment. It may come with an update (as the ability to manually re-arrange spaces in Mission Control).

Waiting for a better solution, you can still use ⌃→ and ⌃← to navigate between spaces (including fullscreen applications).

You can always just use Command-Tab switching, holding tab for the appropriate length of time to reach the app you'd like to switch to.

This sadly works less well with apps that can own multiple full screen spaces, or which can exist as both a full screen window, and another separate window on another desktop.