Yeah, my bet is that this is to blame. Power-Saving Mode is critical for a device like this. If it's disabled (or not supported) either at the access point or in the tablet's control panel, you're screwed.

I've been quite happy with the battery life on my n810, and I leave wi-fi on (with PSM) all the time. I'm not a heavy wi-fi user, but just idling and browsing a few sites leaves me with a full battery even after 16 or so hours off the charger. Normal browsing doesn't seem to have much of an impact on battery life either, since most of the time you're reading the screen rather than transfering data. (Streaming and flash-heavy sites are another matter, though!)

And no worries about wi-fi performance with PSM, either. I've never had a problem opening an ssh session from my PC to my tablet, even when the tablet has been completely idle for hours and is in maximum power-saving mode. It still gets a beacon from the access point a couple times a second, so you won't notice a lag going from idle to active.

Another good battery-saving wi-fi hint is to increase the WLAN search interval. I have mine set to 10 minutes, which means that when I lose the wi-fi signal the n810 only searches again every 10 minutes.

Mind you, Bluetooth might be a killer. I don't have Bluetooth enabled, and I'm not familiar with how it handles power control. If it's enabled and it has a hard time maintaining contact with your phone, that could potentially be a big power drain. (Just speculating.)

In my experience, CPU tends to be the biggest power drain. If you're doing something CPU-intensive (which may include streaming media, depending on the codec) the battery is only going to last a few hours. If the device is warm to the touch, you're draining the battery quickly.