Here's the problem:
If I shoot in continuous mode, I can have several images per second. Aperture will happily import these and rename them with Date @ Time, but subsequently sorting either by version name or by file name will bring results up in mixed order for a given time stamp.
My camera supports the exif field subsectimeorginal and the field shows up in Aperture, but in Aperture, the field is always blank. And there is no primitive to add this to to creating filenames anyway.