Hi,
Yes, we could not attach any event handler to User Information List.
Referring to this article(http://msdn.microsoft.com/en-us/library/aa979520.aspx), we know that “Lists:List events do not fire on the UserInfo list.”
Another asynchronous was discussed in this post below:
http://social.technet.microsoft.com/Forums/en-US/sharepointdevelopment/thread/fb7375ec-eee1-457b-9a3e-4cacec2ebd55
You can consider create a service to monitor the change UserInfo list, or get the modification logs from audit information.
Hope this can help.
Best Regards,
-Aaron