Found a slight conflict with the WangGuard anti-splogger/spam plug-in.
WangGuard displays its own column with option links in the Users page and
egister IP will no longer displays the user's IP address in the Users page under the "Signup IP Address" column when WangGuard is activated.
The primary function is unaffected though. Register IP still records the user's IP address and it's still displayed in the Edit Users page.