I believe iPad is unlocked (at least in US), so you should be able to use any 3G carrier provide UMTS/HSPDA at 850/1900/2100 frequency. As of now, not sure who else did provide service other than Fido/Rogers.
Also iPad did not have voice function, there are some VoIP apps currently available for iPad for online calling such as Skype.
iPad itself does not have voice feature. However as it has the 3G data modem which use cellphone network connect to Internet, therefore you could use VoIP application for calling function. Another problem is the headphone, you have to use bluebooth headset.
Assume Fido have UMTS/HSDPA at 850/1900/2100 MHz, yes, just plug in the sim card.
The difference between iPhone and iPad Data plan is there is not contract term for iPad user.
However, at the time the only UMTS/HSPDA carrier using 850/1900/2100 band is AT&T in US. T-Mobile use AWS band. But iPad user could use the EDGE network of T-Mobile.