FlickrExport for Aperture provides a comprehensive set of features for getting your images from Aperture to Flickr:

Edit photo titles and descriptions, and add tags to them.
Create a new photoset with your photos, or add them to an existing photoset.
Automatically send your photos to a Group Pool on Flickr.
Choose from a list of your Flickr tags and add them to your photos before uploading.
Automatically add an Aperture keyword to each photo uploaded.
Add Flickr photo ID and URL to Aperture metadata.
Replace existing photos on Flickr with updated versions from Aperture.
Edit Geo data before uploading and integrate with Google Earth to locate your pictures.