I incidentally found and used the correct version of the add-on. I found it at Google code. While writing this post I re-googled the add-on so that I could link to the site. Like so.
I discovered that that version had exactly what I missed: a parameter for the calculator function. Thinking I had an outdated version I copied this version and rewrote the code.
It broke. It seems it was voor Google Maps API v2, and I was using V3.
Then I discovered that there are more versions of the add-on library! The new version for V3 did not have a parameter for specifying a function to calculate what style I wanted to use.

Eventually I just overridden the function as so:

MarkerClusterer.prototype.calculator_ = function(markers, numStyles) {
    var index = 1;
    var count = markers.length;

    for (var i = 0; i < markers.length; i++) {
        if (markers[i].alert != undefined && markers[i].alert) {
            // show alert
            index = 2;
            break;
        }
    }

    return {
        text: count,
        index: index
    };
};

When I create a marker I add the

alert

property before adding it to the array that I pass on to the MarkerClusterer. It works like a charm!

It would be nicer (and cleaner) to specify the function when initializing the MarkerClusterer, but I am just glad I got it working like I wanted to.

In the same press release they state that Oracle is violating their contract by not endorsing the use of the TCK to validate the Harmony JVM of Apache. They threaten to vote against the Java SE 7 release.

I hope that they eventually do vote no on JDK 7. I would love to see Apache fork Java. I think that most Java developers trust Apache more to handle Java than they trust Oracle (and other commercial companies).

Personally I would please me to see Apache and Google fork and create a new Java. Give it a new name. Cut the deprecated API. Drop JSR-277 and use OSGi bundles. Start fresh…..

O wait I’m dreaming…