I believe it will have an impact on the email marketing industry. I can see lots of marketer getting worried. I think however that it will have the worst impact on companies that use their lists in a way that annoy users.

If you simply offer a useful service (and I believe I do with the recently launched http://anynewbooks.com) I don't think you are going to be terribly impacted. People will always open emails that they find valuable whether they are newsletters or not.

If all you offer is low quality bacn though, you're going to hate this new Gmail feature.