It tends to be a bit complicated when in the UK. Our American friends have lots of free SMS type apps which we can't use.
As far as I know there is nothing you can do to link your iPhone to iPad so you can receive your phone texts on the pad. That would be cool.
BUT, you don't mention whether you have an iPad1 or 2. If you have a 1 you can jailbreak and install "SwirlySMS for iPad". This enables full SMS capability on your iPad. HOWEVER, be warned, if you dont have a SMS bundle on your iPad data plan (which I don't on Orange) you get charged full network rate for every one you send (10p in my case). You can receive fine. It's just like a giant SMS app! But the problem is of course, nobody has your iPad phone number and if they start to send stuff there, you'll probably want them to send stuff to your phone at other times... Gets confusing. Although it's cool, I find I hardly ever use it now!
Hope that helps.