For me, I have to go somewhere very quiet, and hold the speaker right next to my ear to hear the hiss. On headphones, I haven't found a pair with which I could hear my N900 hiss. (Or maybe I'm just becoming deaf)

This was a positive surprise, both my N810 (which I don't have anymore) and my 1st gen iPod touch had clearly audible noise from headphones, when they were supposed to be silent.

Edit: There already seems to be a bug report (https://bugs.maemo.org/show_bug.cgi?id=7504) , marked as WONTFIX. Seems it's power vs. latency issue.

Edit2: Ok, I read the OP again, missed the point first time. So the hiss doesn't stop after some idle time as it's supposed to? Have you been able to measure if it eats up battery faster?