Ok, I'm quite late on that one, TileMill 0.10.0 was released on September 19th. A reminder, TileMill is an open source map design studio by MapBox. We mentioned several times TileMill since early 2011. James goes to the point where TileMill 0.10.0 'changes everything' and Brian use it as an example in his interesting article on GIS vs cartography, saying TileMill brings simplicity and attainable elegance.
From the 0.10.0 announcement: "This release redefines the creative possibilities for web cartography with its new support for compositing layers and features, achieving photoshop-like clipping, masking, blurring, or highlighting. This powerful set of compositing operations can be used seamlessly across vector and raster layers all using pure CartoCSS. The compositing now possible in TileMill, in combination with image patterns or raster hillshades, can enable effects of uniqueness and beauty that go beyond what has previously been possible."
From the post: "TileMill is a tool for cartographers to quickly and easily design maps for the web using custom data. It is built on the powerful open-source map rendering library Mapnik - the same software OpenStreetMap and MapQuest use to make some of their maps. TileMill is not intended to be a general-purpose cartography tool, but rather focuses on streamlining and simplifying a narrow set of use cases. And how government agencies can use it."