Now that there is a tool to convert an MXD to Tilemill, one must then wonder if the ability to open QGIS documents in Tilemill will ever exist.
This somewhat exists - here's a page of the various converters available. Exporting CartoCSS from other tools requires both mangling the native format and normalizing differences between how they treat data & the capabilities of the renderer.
Definitely open to adding more features/functionality to my QGISTileMillExport plugin (as well as adding it to one of the plugin repos for easier installation once it's a bit more mature). I stubbed it out as a proof of concept for my own use a few months ago.
As for workflow, I saw it as a way to bridge between data/rough styling in QGIS and final presentation work in TileMill, as well as allowing me access to QGIS' more advanced binning and gradient tools (which I figure TileMill will eventually incorporate in some form).
Feel free to mess around with it and submit questions/comments/bugs/requests over on the issue tracker
Also, pull requests welcome! :D
QTiles designed to generate raster tiles from QGIS projects according to the Slippy Map specification  and supports two output types: directory and ZIP-archive.
I'm not sure if this works directly with TileMill, but it looks like a good start.
There is also my QGIS -> Mapnik XML plugin at https://github.com/springmeyer/quantumnik. I wrote this before CartoCSS existed, and I've been planning on updating it to output CartoCSS by leveraging https://github.com/rundel/carto-generator. If anyone else is interested in this approach get in touch by creating an issue at https://github.com/springmeyer/quantumnik/issues/7