I notice a lot of other phones "require" you to get a data plan on the phone to be able to use it and for updates. Since the N900 is basically a pocket computer / internet tablet with wifi which happens to have cell phone hardware in it as well, I'm kinda wondering how necessary it is to get cell service on the device.

Don't get me wrong, I intend to, but I was thinking about just using it with wifi as a pocket computer for a few weeks after I get it. Other than not being able to make cell calls, or access the Internet where there's no wifi available, can you think of anything else that would be limited on it if you didn't sign up for a data/voice plan right away?