I think I've mentioned this before, but whilst everything that has been said is relevant to a stock iPad, it is possible for any iPad with GSM / 3G to send and receive SMS messages. It's just the OS has been hobbled. All the necessary hardware is there in the 3G chip. A quick jailbreak and installation of an app such as "SwirlySMS for iPad 3G", $4.99 from Cydia (the jailbreak app store) will allow you to send and receive genuine SMS messages via the carrier as you would on a phone.
HOWEVER, a couple of important points:
1) iPad data plans never have SMS included. Therefore you WILL get charge the full network rate if you send an SMS this way. In the UK that's 10-15p per message. Ouch! Of course, if you use the SIM from your phone which is activated for use on your iPad too, and which has an SMS bundle you can send and receive perfectly well within your carrier plan.
2) Whilst SwirlySMS is IOS 5.1.1 compatible and works on all 3G iPads, it just is not as convenient as my phone!. The interface in Swirly is not great, and with iMessage existing now I really have all the functionality I need since most of my friends have iMessage!
You can see a really old discussion on it here -
http://www.ipadforums.net/ipad-general-discussions/30544-sms-ipad.html
I appreciate jailbreaking isn't for everyone, although it is relatively risk free, however I do like everyone to be in possession of the facts
Hope that clarifies.