It's an interesting thought, for folks who use little to no voice at all.
However, SMSing is what I would miss. I know there are these apps, but honestly I've never used them (I have unlimited messaging). Maybe someone can fill be in on whether or not those apps would be a viable replacement.
But yes, I think VoIP technology would work great, espcially on a JB iPad that could work with a bluetooth headset. Maybe you've got your laptop in a bag or case, but it can still ring via bluetooth.
Now, onto my personal opinion, no I wouldn't. I like having my cellphone and there are plenty of opportunities where the iPad can stay home.
However this is an interesting idea for a business. For example, if instead of a phone line in my office, I had a Google Voice account. I could then use it on my iPhone, iPad, office computer, etc. I mean as it is now I can forward calls to my cell when I'm away but that uses minutes.. hmm...
It could definitely replace a land-line for some folks. I haven't had a land line in about 8 years, but for folks who like having a "number for their house", using a Google Voice number would be great. Same reason, the iPad if it's usually at the house, or keep a PC or something signed in. Even if just the iPad, it's an inexpensive, occasional use voice solution for something like that.