I too have just experienced this issue with N900 and BH-905 on the first day of using them together on only the second call I made.

Finished the first call and then made another. Somehow the audio went to the handset not the headphones. Tried to make the call again but the headphones went dead.
The headset connection icon wasn't there and there phone indicated no bluetoth connection. I just could not get the bluetooth indicator light on at all and no buttons responded.

The ANC worked and if I used the cable I could hear music from the N900.

Found this thread and the three button reset seeed to work after much fiddling. They now work.

Will see if this happens again and but if it does I will likely return the headphones.

This coulpled with the bluetooth/wifi issue when playing music (using of the wifi interuptions the bluetooth audio and then the music pitch goes funny) really makes we wonder about the quality of nokia software and their testing....

Very disapointed given cost of the phone and headset. There should not be this many issue!!!!