on My new N810 - with the default apps (abnd Claws mail instead of regular email program), I could leve it on and connected to Wifi and the battery drain was minimal - so much so that I only had to charge every other day.

Now that I have a stock ticker applet whcih refreshes every few minutes in getting data, and have modest - which autochecks, the battery drain is more and is noticeable throughout a single day.

So it depends on what you have installed which connects to the networks even in sleep more that the battery drain will matter.

Leaving a browser page up in online state will drain when the page autorefreshes - like a google reader page, or yahoo news page. So that is subjective again.
If there is no significant online traffic in idle mode also, then there is little to no significant battery drain leaving wifi on - I believe.

So far very happy with the battery state on my N810. I only go offline at home and sometimes shutdown at night before I go to sleep.