How can I detect IP addresses that connect to IIS?

The easiest/best thing to do is set up a network analyzer such as Ethereal or, better yet, OmniPeek with an address filter on the local host or on a secondary host that's plugged into the span/mirror port on the Ethernet switch and can see the traffic. Any traffic to or from that IP will be captured.

For more information on securing Internet Information Services:

• Windows security toolbox: Network security tools
  Check out these 12 free tools from SearchWindowsSecurity.com to prevent attackers from crippling your network.
• Keeping your IIS server secure
  Don't allow your IIS file server to remain vulnerable. Be proactive and learn how to better secure it while running Windows Server 2003.
• Internet Information Services Security Journal
  IIS security is improving, but Web server security is still a concern. Find out how to lock down IIS in this journal.