It seems that Google Maps doesn't really support using SVG images for markers at the moment. This fact is easy to overlook, because it turns out that SVG marker images do actually work in, eg. Chrome and Opera.
However, the Google Maps API (v3) specifically provides Symbol objects for displaying vector paths in map markers. I found that specifying the vector image in SVG path notation allowed it to work in IE and other browsers.
Example (from Google Maps docs, here):
var goldStar = {
path: 'M 125,5 155,90 245,90 175,145 200,230 125,180 50,230 75,145 5,90 95,90 z',
fillColor: 'yellow',
fillOpacity: 0.8,
scale: 1,
strokeColor: 'gold',
strokeWeight: 14
};
var marker = new google.maps.Marker({
position: map.getCenter(),
icon: goldStar,
map: map
});
(Thanks to this answer too)