How can I notify my PC every time Android changes its IP?
Rather than a notifier and managing all of this yourself, had you thought about using a Dynamic DNS client? This would allow you to always use the same name to contact your phone over the internet (or even a local LAN) and the 3rd party would manage the problems with changing IP addresses for you.

There are a number of Dynamic DNS clients on the Market, I haven't tried any, but it's a pretty common solution for machines that change IP, so I can't see why there'd be a problem.