Today is the pre-release of version 0.1 lite beta of the Drupal Theme feature in psd2css Online. As you might be able to guess, I expect it to change as I get feedback from the Drupal community. In fact I'm not even going to write up formal documentation until the design has flushed out due to feedback.
Having said that though - right now you can build a Drupal Theme in Photoshop, name some layers for the content that Drupal will provide, convert the PSD file to a fully functional Drupal (version 6.x) Theme, copy it into your existing Drupal install and it works.
