This Tumblr theme implements the hatom microformat. The code for this theme is hosted on github here. Please feel free to add to and improve upon the code in any way you can.
The complete source ready to copy straight into your Tumblr customize page can be found here.
See the README file for further instructions.
Microformats are a set of open standards and specifications for the construction of (X)HTML that is meaningful to both humans and machines.
Definition from the microformats wiki:
Microformats enable the publishing of higher fidelity information on the Web, providing the fastest and simplest way to support feeds and APIs for your website.
See more explanations of what microformats are, and what you can do with them.
Designed for humans first and machines second, microformats are a set of simple, open data formats built upon existing and widely adopted standards.

Thanks to the hAtom format, the content of tumblogs using this theme can be extracted programatically directly from the XHTML. There are some limitations due to the way tumblr publishes audio and video content. I would have liked to have implemented the hAudio format for audio posts but this is not currently feasible.
Here are some links to hAtom to Atom transformers that do a great job of extracting this hAtom formatted content:
Also well worth trying out are these two addons for Mozilla Firefox that help you locate and extract microformats while you browse: