While working on my latest theme I suddenly noticed that the PAGE TEMPLATE selection dropdown menu had disappeared on me. This in WordPress 3.0.4 – first time this happens you’ll scratch your head wondering if you went daft. But no.
The solution: activate another theme (for example the default one), go into a page and most likely you will see the dropdown back where it should be. Now go back and select again the theme you’re working on. Back into a page and tada, it’s all working again.
Hope the remedy is just as easy for you as it was for me.