I had got my hands on both iPad and Android tablet, the one difference I found in both is that you can make phone calls from Android but not with iPad.
Actually iPad don’t support calling or sending SMS from it but if you are using Android tablet then you can do it.
If you are using jailbreak iPad 1 (Jailbreak iPad 2 is not yet out) then you can download and install PhoneIt-iPad from Cydia and use your iPad just as your iPhone.
You can make calls using your normal SIM card which you use on your other phones, you can even send SMS, but MMS support is not yet added in it which we are expecting in near future.
As per me price of this app is little high, it cost one time fee of $19.99.
Things to remember:-
This app support iPad 1 with running iOS 4.3.3 and Apple is soon releasing iOS 5 but App developers are not sure whether it will support iOS 5 or not.
Purchase it from HERE.
Download it via Cydia with repo :-