I think the idea of wholly unrelated cannot be easily avoided, esp. with mega-corps of today that does everything. Imagine if you worked at microsoft - you would not be able to make a game in your off time, because microsoft owns a game studio too. You'd be hard-pressed to find something that is both software, but unrelated to microsoft.
The clause ought to be that your off-hours, personal work must not have a direct conflict of interest with your job.
