Matthew Turland has written up a new tutorial explaining something that isn't "currently very well documented" in the Zend Framework - output filters.
They're mentioned in passing in the Zend_View documentation, but not reviewed in detail anywhere in the Reference Guide as of version 1.5.2. I was curious enough about how to implement markup minification that I decided to trace through the Zend_View source code in attempt to discern how output filters actually worked. As it turns out, it's actually pretty simple.
He explains a simple example of how they work, using a Zend_View object to build the filters on (addFilterPath/setFilterPath), and how he created his own output filter - the Minify filter. A minifier removes the comments and whitespace in the output to help reduce its size.