I just noticed today that you can tag products on Amazon, check out this screen shot:

I wonder if they will be providing access to this through their API? Perhaps Jeff Barr, or someone else over at Amazon Web Services could let us know. You can already tag amazon products on third party sites like swagroll.
Comments
I'm curious why someone would want access to this in an API. I don't think its a bad idea, I'm just wondering what ideas you were cooking up. ;)