Ableton have added a nifty sidechain feature to the Auto Filter plugin. This allows you to modulate your filtering with a routed signal. Just for kicks I’ve put together a little sample of what type of transition effects you can get using this tool.
Sample 1 — This is a couple of loops leading into a new track
Sample 2 — This is a series of 4 or 5 tracks (I forget) leading into one another using various sidechained filtering.