It's not posterization but a relatively new look not using filters in a basic way. Actually the posterize filter isn't used at all in this shot. We have been experimenting at my studio with different variations of this method.
That effect is called rotoscoping and is very easy to achieve in photoshop cs3 but it has it's draw backs. I feel it's been done so many times and that look originated from a movie that was done way before that Keanu movie. One of the first classes I took in 1999 was in rotoscoping for digital video using a software called flame which blows away aftereffects but comes with a huge price tag that no one could touch which sucks. For the past two years I have been developing new digital effects for print ads and actually had the opportunity to art direct national ads for Motorola among some other great projects.