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It could be done, but that would require a phone number. Then someone would want phone service on the iPad. Then you just have a huge iPhone. Most people who have an iPad can use the 3rd part SMS software (like GoogleVoice) or IM software to accomplish theses tasks, without Apple or the carriers having to figure it out.

TEG
 
There are a lot of texting apps in the app store that allow you to text between an iPad and any cellular phone with MMS.
 
Try text plus. It is free in the app store. They give you a phone number. It can access any of your contacts with mobile numbers or you can simply enter a mobile number. Works great.
 
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