The number of users utilizing Uptime Robot is regularly growing and, in order to fulfill this growth, a new monitoring engine will be added to the system by this Monday (2 July 2012).
If you never needed to whitelist Uptime Robot's IPs to make the monitoring work correctly, than it is ok to ignore this message as things will keep working ok.
However, if your monitors/websites are behind a firewall where you need to whitelist the IPs, the new IP is: 188.8.131.52.
And, you can find the full list of IPs being used at: http://www.uptimerobot.com/about.asp .
Have a great day!