I plan to support 1.5.x, but at the moment the Arduino developers are still making significant changes to the platforms.txt, library search and other important details. I'm waiting for a while, mostly because people seem to be pretty happy with Teensy on 1.0.3 at the moment, but also because I'm not excited about having to redo a lot of work for 1.5.x as the Arduino developers change stuff. Hopefully it will stabilize nicely before more people make the 1.0.3 to 1.5.x switch?