Hernán Morales points to a preview on the development of an interface builder for Seaside
The ideas of this package are based on WindowBuilder Pro. Right now they have support for most common web controls (TextField, Panel, RadioButton, TextArea, Label, Button, Anchor, CheckBoxGroup, RadioButtonGroup, ListBox, FieldSet).
Here is a screenshot (click on the image to enlarge):
He thinks the package would be available at the end of this year or first months of 2008. He also points to two preview videos: