The iPad may turn out to be an emergency mobile phone by using VoIP applications like Truphone, Skype or Vopium which are already available.
Out of the three, only Skype allows VoIP calls over 3G networks with UK mobile network 3 UK being the only one openly supporting the disruptive technology and only the two others have native iPad applications.
Both Truphone and Vopium can be downloaded via iTunes for free and Vopium even gives you 15 minutes and 30 free SMS to get you started; you will need WiFi access to use the services (see "4 Reasons Why You Should Buy iPad WiFi Rather Than 3G Version").
All three VoIP services offer both PAYG services and various subscription levels. While calls to local UK numbers and mobile phones are relatively high compared to traditional telcos, they offer much lower calls to foreign countries (try making a call to the US on your mobile).
In addition, Skype and Truphone both offer integrated IM clients and Vopium even offer a remote backup service.
Using the iPad as the phone means that you will need to have a proper Bluetooth headset - like the GN Netcom BT530 Bluetooth Headset - otherwise you'll need to use the device's microphone and the inbuilt speakers.
ITProPortal has also launched a contest inviting prospective and current iPad owners to get up to £500 worth of iPad accessories (including iPad Apps and Apple iTunes Credit) simply by submitting the list of accessories they'd like to have. Simples!