Just for clarity (used to sell em for a bit), the iPhone does not have phone capabilities. You would not be able to use it as a phone either when connected to a wifi network, or with a good 3G signal and a data SIM.
Ignoring the fact you’d look a total plonker doing so, it would be hopelessly impractical, as you’d only be able to contact other Skype users for free, anyone on a “normal” phone and you’d start running up a big bill on Skype. And you’d only be able to use it in areas of wifi or good 3G as said already, which is still surprisingly limiting.
Quite simply, get an iPhone. Does everything an iPad does, only it’s smaller, cheaper, and you can make phonecalls with it too! Ingenious…
Oh, and there’s also a Samsung (forget the name) I saw someone using the other day… How it is classed as a “mobile” phone I don’t know, it was about the size of a 7″ Tablet so barely any smaller than an iPad! Ridiculous if you ask me…