Yesterday, I discussed the suit involving EA Sports and an original designer of the Madden video game franchise. The video game was a work-made-for-hire, because it was created by the employee within the scope of his employment. If he was a volunteer or independent contractor, it may not have been a work-made-for-hire and would depend more on the language of the contract.
In determining whether conduct is within the "scope of employment," courts use general principles of agency. If a work is designed to serve one's boss, it is likely within the scope of employment. For instance, if you are a software developer and write code as part of your job, it is within the scope of your employment. If you are a software developer and you write an unrelated fictional novel on your lunch break, it is most definitely not within the scope of your employment. It largely depends on the facts of each case.