Discussed today during the virtual Ubuntu Developer Summit were about ports of the Ubuntu SDK to Windows and OS X, to assist cross-platform developers that may use OSX/Windows as their primary development environment but would be interested in the Ubuntu app ecosystem.
With limited engineering time and resources, it appears that the Ubuntu SDK for OS X and Windows will end up being a virtual machine running a host -- potentially just not for ruinning apps but also for the development environment. "Running everything in a VM would be easiest, fastest to deliver, and accepted by developers."
The primary development environment for Ubuntu apps themselves is the SDK that's based on the already multi-platform Qt Creator IDE but with various plug-ins. To be investigated though is Linaro's binary cross-compiler for Windows as well as investigating how Jolla intends to support its Sailfish platform on OS X and Windows.
Those wanting to find out more can watch the video below or check out the session page for additional details on bringing the Ubuntu SDK to Windows and OS X.