Google refuses to disclose an actual sender's IP address in the header of messages -- unlike others like Hotmail and Yahoo. Some users think that they are sending mail anonymously when using a web client, but usually that's not the case. GMail is the only major free email provider that keeps the identity of its users completely anonymous.
Here are some examples of the headers you will find on outgoing messages from these free mail services:
X-Originating-IP: [188.8.131.52] (Google's IP)
X-Originating-IP: [65.xx.xx.xx] (My IP)
Received: from [65.xx.xx.xx] by web50508.mail.yahoo.com via HTTP; Sun, 29 Jan 2006 14:30:06 PST (My IP)
This causes Google to be blacklisted rather than the actual offenders when spam is reported. There isn't much that can be done about this until Google attaches the sender IP address to outgoing messages. It will be interesting to see if they ever decide to disclose this information (like Hotmail and Yahoo), or continue to keep the privacy of users by masking sender IP addresses.