This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
You'll need to do a DNS lookup to find the host name for a particular IP address. Note that more than one domain can be hosted on the same machine (particularly for smaller sites), so you may not get what you thought you'd be getting.
If you want to do this programmatically from Java code, check out the DNSJava library.