If you'd like to check your IP address by poking around at the Unix underbelly of Mac OS X, go in to the Applications folder, then the Utilities folder, then double-click on the Terminal. Type ifconfig and press Return. You'll get a lot of network information. If you are using ethernet, you're going to look under en0. If you are using Airport, you will find the IP address under en1.
For our example, we are connected via Airport, so we look under en1. The third row under en1 starts out inet then displays the IP address.