fat fingers? i just hit the power button, down ones on the dpad, then center dpad (offline mode). then its power button and center dpad (lock screen) before i but the N800 down.
i guess its a bit harder on the N810 tho as one have to extend the keyboard to get at the dpad...
want to go back online? fire up whatever i want to use (browser, mail, rss for example), hit refresh or select page i want to go to, hit center dpad on the message that pops up, select network in dialog (thanks for not fixing that one, nokia!).
and even then im perfectly able to use the nail on my index finger as stylus for most ui tasks where the thumb or full finger would be to large.