I can also use the voice command on the phone screen (lower right icon) or use the earpiece to initiate. Both work.
When you initiate using the earpiece, you should be getting the Nuance voice system (pops up an entirely black window with white text, covering the entire screen).
Long-pressing the spyglass icon gives you the Google voice search, which is a "pop-up" window that doesn't cover the entire screen and has a help and cancel button.
Correct?
You can trigger the Nuance system by double-pressing the Home icon (most people don't know this and it isn't well documented). You can change what a double-tap of Home does in the settings though.
From what I can tell, the Nuance system does always work through Bluetooth, although on my headset it is VERY VERY quiet. I have a Plantronics Voyager 510. It was picking up voice from the headset though as I had covered the mic hole on the phone with my thumb.
I haven't tested the Google system with the headset yet, but the headset's "voice dial" button is triggering the Nuance application anyway (not Google's app).