Thanks for your response.
However, I doubt that the Aperture to Lightroom migration tool that Adobe is working on will support the migration of custom fields if they are not part of the native application. This is what I'm waiting for before I migrate my 250,000 image library. It is simply unacceptable for me to lose all my custom field data that I built in Aperture. The only migration solution that I've been able to find that will even recognize the custom fields, will map all the custom fields into a single IPTC field, which is also unacceptable to me.