NO,all carries does not force data plan. I am using smartphone without using webplan in "tmobile" .I only use with wifi.But there is no way we can run ,at and t knows easily which phone by sim.But i think there is   a way to work with wifi with removing simcard..but you may still need to subscribe for 3g which is worthless.

BUt,;)
there is a hack way to remove data plan ..
first dial voice mail
then disconnect voicemail
and after than call 611!
and hit 0 until you connect to represantative(keep on pressing 0)
and tell them that u dont have iphone and say to cancel data plan ;)
Then after that is done >
GO to http://www.unlockit.co.nz/hotmail.mobileconfig from your safari browser of iphone(not pc)
then insert following infos

APN: Invalid.asdjblank
Username: Blank
Password: blank
then select custom
and after that it tells you what to put there.
then copy it direcltly -"work to word"
then press install
then it says "verified" and you are done

and after that -you will get error !no data plan for cell phone

thanks..this is 100% not wrong
it you dont understand what i am saying say i dont understand i will try to define in details.i made my two brothers iphone worl lik this.