The following two modifications of the "Selection" example from Component Showcase should behave roughly the same:

---
                    <ice:selectManyCheckbox id="SlctLang"
                            value="#{selectionTags.selectedLanguages}"
                            valueChangeListener="#{selectionTags.effectChangeListener}"
                            partialSubmit="true">
                        <f:selectItems id="SlctLangItms"
                                        value="#{selectionTags.languageItems}"/>
                    </ice:selectManyCheckbox>
                    <ice:commandButton value="SelectAll" actionListener="#{selectionTags.selectAll}"/>
---
                    <ice:selectManyCheckbox id="SlctLang" layout="spread"
                            value="#{selectionTags.selectedLanguages}"
                            valueChangeListener="#{selectionTags.effectChangeListener}"
                            partialSubmit="true">
                        <f:selectItems id="SlctLangItms"
                                        value="#{selectionTags.languageItems}"/>
                    </ice:selectManyCheckbox>
                    <ice:checkbox for="SlctLang" index="0"/>
                    <ice:checkbox for="SlctLang" index="1"/>
                    <ice:checkbox for="SlctLang" index="2"/>
                    <ice:checkbox for="SlctLang" index="3"/>
                    <ice:checkbox for="SlctLang" index="4"/>
                    <ice:commandButton value="SelectAll" actionListener="#{selectionTags.selectAll}"/>
                </ice:panelGroup>
---

The semantics of the action selectAll is given by the following method:

    public void selectAll(ActionEvent e) {
        selectedLanguages = new String[] { "COBA", "Java", "C", "C#", "C++" };
    }


While the first example works as expected - the button "checks" all boxes - the second one only updates the panel with displayed values, but the checkboxes' status does not change.

I do not know (and I am eager to hear) about any workaround to dynamically modify checkbox status.