It isn't the first time ... refactoring a module (well actually I'm transforming and extensions which was incompatible with another installed one) I realized that only the admin panels were working, the product page nearly disappeared: header and footer plus sidebar but no product contents at all.

After validating all the mappings, blocks etc etc it ended up that I accidentally reformat the config.xml ... a new-line was dropped after the class name !

Magento's XML parser fails to match the class name resulting in the empty page.