Adobe has published a six page pdf document on how to migrate from Apple Aperture to Lightroom:

At Adobe, we’re working on a migration tool to help you bring your photos into Adobe® Photoshop® Lightroom® from Aperture, but if you’re eager to switch before the tool is ready, this guide can help ease your transition. 

The migration is likely needed for professional photographers, who will need Lightroom’s power in their workflow. The rest of us can wait for Apple’s migration path to .