After teasing the development of a Lightroom plugin for importing Aperture and iPhoto libraries, Adobe has released the first version to the public. You can read about it on their Lightroom blog.

"The plugin allows Aperture and iPhoto customers to migrate their images and key metadata (such as keywords, events, project structure) into Lightroom catalogs in a seamless way."

Basic instructions are in the blog posting and the import plugin can be downloaded here.

Lightroom Queen Victoria Bampton has much more detailed instructions on her website. I would advise reading her article before attempting a migration. She explains what info will transfer and what won't.