@cway: if you're adding this with hook_form_alter(), you can change its positioning using weights. If it's simply a field to enter in a year, I'd recommend the CCK module to ya.

@ystanley: in order to avoid an outright dependency on CCK, these default product fields are handled separately as you've found out. However, instead of hacking the core module, you can instead write a custom module that uses hook_form_alter() to reorganize and adjust what fields are displayed to the user.