I never had a MID, but I had (still has) a PDA that I tried to make into a mobile, networked computer. But the only way I could use it was to connect to a wi-fi access point, quickly download new email (and send pre-written, queued ones), or quickly browse for something. Then shut down the wi-fi connection. And turning off the screen would in any case turn off a connection. wi-fi could only stay up up to 20 minutes before the battery was empty.

My N800 is a totally different story. I'm online all the time if there's a wi-fi access point nearby, with Skype, email, google talk etc. connected and running. When my battery was still fresh (it's 2.75 years old now) it could stay connected like this for up to three days (and nights). The battery is not as good anymore but it still doesn't make much difference to be online compared to being offline. This is indeed very very different from some earlier "mobile" devices.