This is something I do a lot.
It typtically involves lots of masks (brought in from AI or PS) that I do one of three things with:
1. Animate their expansion (be careful that you’re not using OpenGL preview with this – but adaptive – it’s buggy) I animate it from a negative value back up to it’s default.
2. Animate the points on the mask it self.
3. Animate a new mask on top set to intersect in order to get the reveal on (this is the quickest way – but less elegant)
I wrote tutorials for it in Rich Harrington’s new books AE @ Work and Broadcast Design on the Spot and some examples at http://www.xeler8r.com
hth!
// jayse
Ultimate After Effects Video Training CDs

Free Photoshop CS Video Tutorials
by jayse!