Your post got me wondering so a did a quick test with these results...
With a freshly-charged one-month-old battery, bluetooth turned on (but not in use), display at full brightness, volume up all the way, GPS turned off, continuously playing Youtube videos (from a big YouTube playlist) over a wifi connection (access point in same room), the battery lasted 3 hours. Specifically, I checked on the device at the 3-hour point, and observed a "low battery" message flashing in the corner of the screen; I didn't let the test continue until the battery was actually completely gone. Note that the device seems to know when videos are being played and doesn't dim the screen after 2 minutes like it normally would, so the screen was at full brightness for the whole 3 hours.
Normally, though, I charge the device every night and it lasts all day. Also, when using GPS on a long trip, I leave it connected to the car charger.
I purchased my 810 two weeks ago (brand new, new battery, etc). I permitted the unit to charge fresh overnight, undisturbed. (full 10 hour charge, without turning the unit on, or otherwise disturbing the unit).
I downloaded the Nokia Video Conversion application, and installed on a Windows Workstation. I also obtained a DVD to WMV conversion software package, and installed it a Windows Workstation.
Using these two software packages, I converted the full length movie "Terminator" (Arnold Schwartznegger, Michael Biehn, etc). I placed the movie on an 4GB SDHC card and installed into the 810.
With the 810 configured: max LCD brightness, GPS off, volume 80%, and networking off, I was able to play the entire movie back with occasional short (30sec) interruptions. After the end credits, the battery indicator had only one bar left, and a short time later (10mins), the battery life went down far enough that the low battery warning indicator came out.
I had to shutdown and charge the unit up again.
Battery life seems OK. As usual, it would be nice if it were another hour longer.