Somehow I think if you really didn't use the N900 for anything other than phone, you'd get very similar standby times as a device with symbian.

Disable or pause all the processes that aren't needed on a "phone" (pause the browser process, disable tracker, etc, etc); keep wifi radio completely off (driver modules unloaded), etc.

I'm sure you could code a status menu applet or something (or Queen Beecon Widget or whateter) to go into "phone only" mode where a bunch of non-phone processes are gutted.