Please note that this is a free service, provided as-is with out any warranty.
I know the links have to be consistent so they will probably never change, I'll also try to keep the uptime as close to 100% as possible, but again, I can't offer any warranty for anything and I reserve the right to do whatever I please with it, so you're using the service at your own risk.
If you're planning to use this service in an application you might consider using this link instead: http://ipecho.net/plain
PS: Please don't abuse the system, try to cache the IP for a reasonable amount of time before doing another request.
PS2: This limit shouldn't hinder development, but there's a limit in place now, because the above suggestion appears to have been lost on some users:
Every IP that has a request rate of 10 calls / minute over a period of 30 minutes. Will get blocked for 24 hours.
Although the limit is clear, it would still be in everyone's interrest if you could cache the IP for a longer period of time.