Last year I posted a note on how to move master pictures from Aperture to Lightroom and transfer any metadata that you had entered. Essentially you used the Export Masters command and told Aperture to put the metadata in XMP sidecars. This worked fine for raw files, but not for originals whose file formats were publicly documented such as DNGs, TIFs or JPEGs. Adobe (rightly) expects metadata to be embedded in the file and not in a sidecar, so Lightroom or Photoshop wouldn't read any Aperture captions or keywords in those files. There was a workaround, but it required you to keep your head screwed on.
No longer. I've just had it pointed out that Aperture 2.1's Export Masters command now has an extra option - writing the metadata to the file. So you could just select all the files, and choose that option. It embeds the metadata in raw files too, which you may or may not like. (If not, run Export Masters twice with the sidecar option for raw files and the embedded option for the rest).
It strikes me as odd that they should give priority to this feature. Apart from being limited to the Mac, Aperture still doesn't read xmp sidecars upon import, so there can be two barriers to moving to Aperture. But yet they now make it easier to move your pictures and metadata away from Aperture and to Lightroom. An odd commercial decision, but maybe I should learn to “Think different” ?