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.
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.