Doh! I guess you have to use the space bar. Here's the source.
<!-- language: java -->
protected static final List<KeyBinding> BUTTON_BINDINGS = new ArrayList<KeyBinding>();
static {
BUTTON_BINDINGS.add(new KeyBinding(SPACE, KEY_PRESSED, PRESS_ACTION));
BUTTON_BINDINGS.add(new KeyBinding(SPACE, KEY_RELEASED, RELEASE_ACTION));
}