CHtml class uses get_class() function to render active forms and inputs.
get_class() returns the fully qualified name of a class, so its namespace is included in the returned string.
In order to use namespaced models, you must replace get_class() calls by something like getShortClassName() :
function getShortClassName($object) {
$parts=explode('\\', get_class($object));
return end($parts);
}
This must be done in CHtml::resolveName method but also in classes that generates client validation code (maybe in CValidator and children, I haven’t searched).
For my part, I prefer to wait for a new Yii release, and stay with models without namespace.