Roto Brush tool, once dubbed the holy grail of compositing. The jury hasn't even met yet, but it sounds exciting. See After Effects CS5: The Roto Brush, as well as his interview with Michael Coleman.
For some technical background, see the SIGGRAPH 2009 paper, Video SnapCut: Robust Video Object Cutout Using Localized Classifiers, and Interactive Video Cutout.
Update: Todd Kopriva commented on matte chatter at Creative Cow, "The magic in the Roto Brush effect is in its ability to see across time and make the matte "temporally coherent"---i.e., smoothly animating across time.
There's also a lot of new technology in the color decontamination [new in Photoshop Refine Matte too -ry] and motion blur in the mattes so that the background color is cleaned out of soft edges and such. That part is also available in the separate Refine Matte effect, which is very handy outside of the Roto Brush workflow."
Also, Mark Christiansen also has some AE CS5 training up on Lynda.com, including movies on Roto Brush and Refine Matte, which by the way are explained in detail in the text of AE Help.