You don't need a SIM card. I use my N900 this way at the moment, since I'm almost always in WiFi range and get extremely cheap CDMA service, so I haven't switched to a GSM carrier yet.

You won't be able to make cellular calls/SMS without a SIM of course. You can still use VOIP and do everything else with the N900. It may take longer for the GPS to establish a fix without the AGPS support from a cell tower, though. Generally takes me up to 5 minutes with a clear view of the sky.