If you stop the cellular service, you'll get an error message saying that your data service is off (or something to that affect). This message can be turned off, BUT, it will turn off your WiFi as well. Apple and Cellular service providers are in bed together on this one. They have designed it so you must have the cellular service turned on or get this annoying message every few minutes...or...turn off your WiFi too. I haven't found a fix around this, because there ain't one! I'm stuck with mine. Got ipad wifi + cellular just in case I wanted the cellular later.
If I'm wrong here, someone correct me. I like my ipad, but I'm getting annoyed with strong arm nonsense from Apple. If there is no reasonable fix for this, then yes, you (and I) may have to buy an iPad w/ just cellular and if thats the case Apple S U C K S.