Here's more info about the library spec.
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification
The library spec has been a hotly debated issue on the developer mail list, and for full disclosure, I'm one of many people who's participated in that conversation.
You can also see rev 2 of the library spec is due to be released in 1.5.6. The spec itself was only updated 25 days ago, and the code written only 18 days ago. So far, it's only on github and in the nightly builds. 1.5.6 will be the first beta release with the new library spec. Progress is being made, but it's a slow process.
Of course, it's still not perfectly clear if there will be a rev 3. I believe rev 2 addresses most of the concerns people raised, but it also rolls back some of the features people wanted, which I and many others feel weren't implemented well in rev 1.
This question comes up from time to time, why PJRC isn't supporting the very latest Arduino versions. The fact is Arduino 1.5.X is making some important changes internally. It just isn't worthwhile for PJRC to invest engineering time into supporting a "moving target", where upcoming changes will necessitate redoing a lot of work. Teensyduino would work exactly the same as it does on the stable 1.0.5 version, so the only advantage to supporting 1.5.X would be convenience for people also using Arduino Due or Yun. Today, with important infrastructure within Arduino changing, it just doesn't make good sense to divert time from projects like the audio library.
Hopefully this thread can serve as a definitive answer. Well, at least until the Arduino Team finalizes this spec and other important design features in 1.5.X. I definitely will support new versions, perhaps even before the Arduino folks call it stable, but right now is not the time.