We created a Google Maps GPX Viewer including a little-known AJAX version which with a little tweaking could drop the dependence on a PHP proxy.

This could be rolled into Mapstraction quite easily, except that Yahoo's AJAX Maps don't support polyline drawing.

Perhaps this could be implemented as a custom overlay with Explorer Canvas or similar? (Also, Explorer Canvas imports VML namespaces and behaviours using javascript - we should do that too.