Yes I would think so. I've used both my Blackberry Bold and Nokia N82 without SIM cards for the first few days and had no problems surfing over wifi etc. I would think the N900 would be the same.

For those interested in other phones, I can never get any of my Sony Ericsson phones to work - even in offline/flight mode - without a SIM card... very annoying ! Especially if you are taking a phone overseas just for WiFi use.