Although some developers of raw converters might have you think otherwise, the DNG format isn't just for Adobe. Photographer and Aperture user Micah Walters has written a DNG Export Plugin for Aperture which uses the Adobe DNG Converter's command line interface to generate DNGs from raw file.

Right now it creates a bare DNG with no metadata, but wouldn't it be neat if you could write metadata into the freshly-minted DNG? For example, one might create an xmp sidecar file from Aperture metadata, use it to get the metadata into the DNG, and kill it afterwards. I'm just thinking of IPTC initially, but one might even pass Aperture's adjustment details to the DNG. On the one hand this might allow you to create the DNG with approximate ACR adjustments, starting with some of the most important like exposure or white balance. But it might also be a way of backing up Aperture adjustments into the DNG, and a subsequent plugin might read those adjustments back into the library. The DNG would carry adjustment values for both Lightroom and Aperture. You may say I'm a dreamer….