It turns out that the actual conflict was caused by having not merged the 1.5.6 template files with the live template. There are several changes between 1.5.1 (when they were last merged, in this case) and 1.5.6 which are no longer completely compatible.

So, boys and girls, remember when performing an upgrade, you need to check your template files for any custom template against the changes in the template it is modeled from...