From this DAM and Aperture thread at the Open Photography Forum:

Sidecar Hell is only one issue, but a panacea for it already is to use the DNG format and store both the informational metadata and the editing metadata in the extensible XMP. There's no real reason why, just as Adobe stores its camera raw settings in the XMP, workflow processors like Aperture, Lightroom, LightZone, and Capture NX (there be more) shouldn't do the same. Then your editing instructions aren't hidden from other programs and at least some of your input, like setting WB, will be readable elsewhere. We shouldn't expect all the editing instructions to be meaningful, but if I develop a film in Ilford chemistry then why shouldn't I be able to do print in in Agfa (what if Apple went bust?). Just as it would be crazy for informational metadata to be readable only with the camera makers' software, so in the age of the workflow processors we should expect our editing instructions - our work - to be our property. It's a whole lot easier to commit yourself if a program offers a prenup guaranteeing you can leave with what's yours.