Sebastien has checked into SVN to both the GDI+ and System.Drawing modules an implementation of the binary operations on non-rectangular regions.
For a long time Mono's System.Drawing API stack could only do binary operations on rectangular regions (ANDing two regions, ORing two regions, XORing two regions). With Sebastien's commits it is now possible to use these operations with regions created from any GraphicsPath object.
These operations are used by many graphics controls and advanced graphics applications. So this is a pretty important step for Mono.
His summary post is here (includes links to screenshots).