Well, there is still reprojection done on-the-fly. For instance, when you overlay a set of points or a polygon on a Google Map, these points have to be reprojected to be overlaid. This is done in JavaScript on the client side.
There are two things going on here. Raster images that are overlaid must be in Google's Mercator Projection, correct. However, when you provide shape data (polygons, points, etc) to Google's API to be rendered on the client, the JavaScript code transforms the points to the screen projection before being overlaid.
