This is a basic proof-of-concept package that uses the _uihooks for Meteor Devel.
What it gives you is a basic {{#transition}} helper -- which if you wrap around an {{#each}} or a {{#if}} will animate the removal / addition (and eventually movement) of elements using CSS Transitions.
Check out the basic-example for a simple usage.
You must have a transition defined for the element on the .off-screen -> not .off-screen class change. If you don't, there be dragons.
Eventually this will be a real package with options and the rest. Until then the best contribution is suggestions to what it needs. Perhaps it's perfect as is?