actually, it’s more complicated than that. i figured out a way a moment ago but now i’ve forgotten,…it’s ok, i will remember again,…just give me a moment.
<edit> ok this is the thing,…you have to make an empty, parent the lattice to the empty, use the empty to control general movement, and set RVK’s ( relative vertex keys ) to control the lattice distortion. there is no way that i know of to actually keyframe the influence of the lattice, so it must stay with the object. someone else may know a different way though.
<edit> actually the empty isn’t necessary, i guess the only important thing, is you need to learn how to animate the vertex of the lattice. for that, there is a good tutorial at the blender char site, which is listed at the top of the animation forum in a sticky thread.