zen_preprocess_page doesn't get called for subtheme

The most recent Zen code has switched from phptemplate_preprocess_page to zen_preprocess_page in template.php because "phptemplate_HOOK is nonsense with sub-themes." But zen_preprocess_page doesn't get called for my subthemes while phptemplate_preprocess_page was getting called in the earlier version of Zen I was using. This means some setup I expect to be happening isn't happening, notably the wireframes and block editing CSS -- though that has a separate problem which I'll log as another issue with a patch.

If I rename zen_preprocess_page as phptemplate_preprocess_page, it gets called. I tried rebuilding the theme registry with the zen_preprocess_page function but it still doesn't get called as far as I can see.

The Drupal API doc for Default theme implementations says:

While this can vary based upon the theme engine, the standard set by phptemplate is that theme functions should be named either phptemplate_HOOK or THEMENAME_HOOK.... The ENGINE_HOOK() syntax is preferred, as this can be used by sub-themes (which are themes that share code but use different stylesheets).

Garland, for example, uses phptemplate_preprocess_page, not garland_preprocess_page.

That implies that only one THEMENAME_HOOK is called... the one for the current theme and not the base theme. However, this doesn't seem entirely consistent as zen_breadcrumb() seemed to be called in my subtheme when I tested it.

I can't tell from the Preprocess functions doc whether multiple THEMENAME_PREPROCESS functions are supposed to be called or not. It kind of implies there could be a cascade of them when it says:

Although impractical, it is possible to grow this list beyond the ten shown above through sub-theming by adding strictly through the theme name as it is shown in the last two examples.

I'm wondering if Zen should also go back to using phptemplate_preprocess_page instead of zen_preprocess_page so that subthemes don't have to reproduce Zen code in their preprocessing code. Or maybe zen_preprocess_page is supposed to get called for my subtheme but is not for some other reason.