For all of those too impatient to wait for a full Sailfish OS binary SDK release (due for Q1 2013), the Mer project has released a good documentation on how to build your own environment under VirtualBox.

And it's here:

https://wiki.merproject.org/wiki/Platform_SDK_on_VirtualBox

You will need a Linux box and some extra RAM, and you're ready to write some Qt apps for Jolla.

Nixman