If you have an iPhone there may not be a compelling reason to get an iPad with 3G. You could probably get most of your information while on the go with you iPhone. Wifi is so ubiquitous now that even if you are traveling you will be able to conveniently find a hot spot at McDonalds, most coffee shops, many restaurants, almost any shopping mall, most all public libraries, bookstores, most hotels, highway rest stops, hospitals, campuses, many homes, private campgrounds, etc. In those places you can usually sit down and work and I think the iPad will be much more convenient to use than a laptop computer in those situations.
I'm an RV user and west of the Mississippi River the Verizon Wireless ads are true. 3G is rare when on the road but wifi is generally available at the many McDonalds that dot the highways. Edge is usable for getting some information on the iPhone but is painfully slow to use interactively. If you vacation in national and state parks, national forests and other wilderness areas both wifi and 3G might not be available but wifi usually is at the nearest town.
Also, you need to consider your use. If you have it loaded up with data, books, music, videos, etc. then you may spend most of your time with the iPad off-line.
Again, if you have an iPhone I suspect you could get by without 3G. Why go back to a dumb phone? The iPhone still has advantages over an iPad when out and about. I would consider 3G if there were no monthly rate but a data demand user fee. I would probably not even use the $14.95 rate most months same as I don't currently use text messaging on my iPhone enough to pay extra for unlimited use now.