The loader seems to have an undocumented feature, in that the API key
can be set after including the JS source.
This works for me:

<-- load the Ajax loader w/o API key -->
<script type="text/javascript" src="http://www.google.com/jsapi"></
script>

<script type="text/javascript">
        var gKey = '';
        switch (window.location.hostname) {
                        case 'localhost':
                                gKey = 'ABQIAAAA0cEo...';
                                break;
                        case 'www.example.com':
                                gKey = 'YBeYhAA0cEoDf...';
                                break;
        }
        google.loader.ApiKey = gKey;       // UNDOCUMENTED feature
        google.loader.KeyVerified = true;   // not necessary, but defaults to
false otherwise(!?)
        google.loader.LoadFailure = false;  // not necessary, but defaults to
true otherwise(!?)
        google.load("maps", "2");
</script>

Setting the KeyVerified and LoadFailure values does not have any
effect in my tests (these seem not to be evaluated when loading the
map api).

Is this supposed to work the way it does and can we rely on this
behavior for production scripts?