Chris Meyer posted the last of the 12-part series Deeper Modes of Expression, based on an extra in Creating Motion Graphics with After Effects, which was recently updated for CS5.
The 12th installment is Deeper Modes of Expression, Part 12: Expressive Text on ways to manipulate numbers and text using expressions:
"Finally, we’d like to demonstrate ways to use expressions to manipulate text created with the new Type tool. We’ll show how to display time and parameter values as text, both for diagnostic purposes and to create graphical elements. On the second page we’ll show to how to manipulate strings of text, including using the Buzz Words animation preset to cycle through words in a list. Then on the last page we’ll talk about the Expression Selector to animate text created with the Type tool."