Can't remember what aswalla posted, but for posterity's sake I'll mention that the osCommerce method of storing passwords involves using "salt" on the password... appending some random characters and making an MD5 of it all. You can't undo MD5 hashes to get the original password, unfortunately, and Drupal just uses an MD5 of the password itself. The only way to preserve customers would be to implement a custom user validation that preserves/uses the osC method... and this would get messy pretty fast.

You can at least migrate the customers and let them request new passwords when they want to login.