Chris, thanks for bringing this up.
Next version of S3fm will allow just that: sharing files and "folders" with your friends and colleagues using your own S3 account. A bucket owner will be able to use his or her credentials to create new (non-AWS) "accounts" and assign different permissions for each user. Then s/he will be able to select files to share or "folders" for uploads for each of those users.
A secure authentication method has been developed on top of regular Amazon S3 API so no 3rd party service will be required for that purpose. In fact, your newly created account credentials are not even accessible to anyone but you and you users . On the flip side, if you loose them - they are gone, we wont be able to restore them. :)
This version was expected this coming Fri (Aug 9, 2009), but apparently will be delayed another week or so.
Happy to help, feel free to follow up with questions or ideas,
Alex