themeName_preprocess_page not getting called for subtheme

To reproduce, create a subtheme of Garland called subgarland and add a preprocess_page function:

1. create directory subgarland in sites/all/themes
2. create subgarland.info file:
name = Subgarland
description = A subtheme of Garland
core = 6.x
base theme = garland
3. copy page.tpl.php from Garland into subgarland folder
4. create a template.php with a function subgarland_preprocess_page that sets a new variable.
5. edit page.tpl.php to use that variable.

Note that variable doesn't get set and isn't available for use.

Preprocess functions doc says that themeName_preprocess_hook runs at the end of the preprocessing phase. But I discovered in working with Zen subthemes that subtheme preprocess_page didn't get run, and neither did the Zen preprocess_page, unless it was named phptemplate_preprocess_page instead of zen_preprocess_page. See #246231: zen_preprocess_page doesn't get called for subtheme.