The android OS is only one part of the hero's firmware. Below it you have device drivers for the phone's hardware, and sitting on top of it you have HTC's SenseUI front end.
HTC have to integrate the rest of this stuff into the android 1.6 OS (drivers no big job, sensui is a bigger issue) before it can be released. Rumours recently abound that they wont be bothering with 1.6 and will jump straight to 2.0, so you may have a bit of a wait (but it will be worth it, cos 1.6 doesnt really offer anything to get excited about over 1.5)
Edit: wow, i took a very long time to post this, and in the meantime i got usurped.