I'm also curious, is it practical to have it always online in the most minimal sense:

How long would the n900 off charger if you just left it idle, but connected to IM and one 5 minute IM chat per hour. No other activity

Hows the battery life for

connected via WIFI only, but GSM radio enabled for phone calls - uses the battery least, but is not practical if you're mostly on the move or have no access to wifi

connected via 3G - overkill just for IM

connected via EDGE - can this minimal case last 9am to midnight? since the marketing slogan is "always online"?