The HiTechnic Gyro Sensor is now available. You can read more about it here.
The price is US$48.99 and can be ordered directly from the HiTechnic website. Because of HiTechnic's relationship with LEGO, it is also anticipated that you'll be able to order the sensor from LEGO shortly.
HiTechnic has also made available several downloads, including some sample programs PLUS the NXT-G programming block. You can get all that here.