I use ElasticEmail to send my newsletter and they have a way built in to their service to track delivery, bounces, etc but for html email they also can track if it was opened. So you could use that if you need an immediate and easy solution. I don't have any immediate plans to build something special for that. I suppose one could use an iframe that isn't visible to load a page from their site and track that.
To me the most useful thing though is the tracking of link clicks in my newsletter which I do by tagging the links from the newsletter to my site with google analytics ie by appending "?utm_source=mojoNews&utm_medium=email&utm_campaign=mojoNews" to the urls. That gives me a way to report on how well my newsletter is doing at driving traffic back to my site.