I see 0.0.0.0 as source IP from a visitor.
[attachment=1331:ip0000.jpg]
Since some ip addresses are displayed correctly, the possible causes in your case are an ipv6 address (not currently supported) or ip spoofing using proxy headers (made possible via an increasing number of browser addons).
I already created tickets for these. If you have suggestions (or patches) to address these tickets, please help in improving Piwik.