We built our Web Consistency Testing service on top of Selenium and provide an API for integrating it into your Selenium testing suite. I assume you're looking to build your own rather than use a third party. In that case, there's really two parts that complementary to Selenium that you'll need to deal with: getting the DOM values and getting the screenshots for visualization.
For the screenshots, you almost certainly want the full canvas ones. Selenium provides this for Firefox out of the box. If you use the WebDriver protocol, you also get it in IE out of the box. If you use the RC protocol, you'll need to check out SnapsIE to get the capability for IE. We're pushing on the Chrome team to add something to their API to make support for this out of the box much easier -- until then, you likely would need to produce your own NPAPI-based plugin.