Just tried this out. Very simple and friendly sign up and setup. Looks great so far and seems to serve its purpose well. I will mention though that it felt odd having a link in the main navigation (Malware Scan) take me to a different website. Having not been familiar with Sucuri to begin with, I wasn't sure if it was an advertisement and how it related to UpMonkey. Thanks very much!
Seems to work fine, but I got this when I added one of my sites:<p><pre><code> 2012-11-30 03:10:25 OK: Site added: http://example.com (is up).
2012-11-30 03:10:25 Warning: Site http://example.com is down.
</code></pre>
I.e., it thought it was down, and notified me accordingly, but then shortly afterward it recognized it as up (which it was the entire time).