I normally switch around my virtual desktops using the ctrl+num. Now, Exposé shows all the desktops and the fullscreen app whereupon clicking the fullscreen app rectangle will take me back to it. I'd rather do so via keyboard nav. cmd+tab doesn't work for me because I'm doing this with VMware where I have multiple windows open, one is fullscreen, others are not. cmd+tab seems to land me on whatever VMware window it randomly chooses.
You can configure these shortcuts in
I think the default is control-option-left/right arrow, at least that's what I have it set to.
In mission control, you can also re-order the position of the desktops/full screen apps by dragging them around. I think by default they're ordered by most-recently used desktop, but you can change that (I can't remember how, that would be a separate question anyway).
By default it's Command+Control+F.
However, it doesn't work in all apps. Some don't do full screen and some pre-Lion apps retain their original full-screen keyboard shortcuts (e.g., Command-Shift-F in Firefox).