Google changes their layout...
Google made a change to their layout today, as websites often do. Understandably such changes have a tendency to break user scripts or cause unintended behavior.
At this point, Google's latest change causes the "Find My Site!" button to not show up on the initial search page (http://www.google.com/). It does, however, still show up and even function just fine on the search results page (http://www.google.com/search?q=find+my+site+use...).
You might have a difficult time finding the button at first, but if you look closely, you'll see two new blue magnifying glass buttons just to the right of the search box. The one on the right is the "Find My Site!" button.
Hold tight for updates to the script.