Your IP address is broadcasted to every website you visit. In fact, you can even go to a site such as http://www.whatismyip.com/ to find out your IP quickly and easily.
Yeah, it's pretty weird to see it show up in a post the way I worded it, but nevertheless, this IS the Username topic...

No hacker will get any data by sniffing that post that they don't already have. The packet containing that post that's sent to your computer... guess what, it's encoded with the IP directly in the header already.

Does not the postman see your home address with every letter you get? He has to, or he can't deliver it. It's the same thing here.
So really, you can relax. There's no harm being done.