Go to "Window" -> "Preferences..." -> "Pydev" -> "Interpreter - Python"
In the "Forced builtin libs" section, click "New..."
Type in "PySide" and click "OK".
Click "OK" to close the Preferences window.
How to get code completion for PyQt using Pydev, http://www.saltycrane.com/blog/2007/06/how-to-get-code-completion-for-pyqt/