Simply opening and closing the editor didn't seem to have any effect in my generated project, but in a "New Seam Project" result, it seems to work as described there. Strange.
Another option to fix it is adding http://jboss.com/products/seam/pages name space to XML Catalog in preferences like it described below:
1. Open Preferences dialog and find "XML->XML Catalog" category.
2. Press "Add"
4. Fill "Key" as http://jboss.com/products/seam/pages-2.2.xsd
5. Close all dialogs by Ok button
After that any new Seam 2.2 project will be created without error.
For existing errors you need to invoke 'Validate' action from context menu in resource tree for whole project or particular file.
Cleaning the project or whole workspace works as well because it triggers validation.
thx denis for your workaround.