To illustrate the problems with this approach, there was a Linux-based media server that stored its free trial timestamp in /usr/bin/.tv. It only takes an strace for someone to realise that file is being accessed - in this case, simply deleting the file restarted the trial.
If you are a single developer you will have to spend a lot of money and/or time implementing a protection scheme that only needs to be cracked by one person to be available for everyone. Of course, your aim may only be to deter casual software pirates, in which case even the most basic protection (such as the method described above) will do the job.
