I would not consider an ipad without 4g. I previously had an ipad with wifi only, and it is much more useful for me with cellular.
The primary benefit of a tablet is the small portable form factor, long battery life, and large touch interface. The easy portability lends it to be used in many situations. Instant cellular capability increases the any time usefulness significantly. It is a big fucking world and much more of it is covered with cellular than easily accessible wifi. Tethering to a celphone is inconvenient and sucks batteries. Further, the ability to remotely use my home or work computer nearly anywhere magnifies the functionality of the tablet.
For those that say a tablet is not a business tool, they are simply closed minded. Like any tool, it has areas where it excels, and areas where it is weak. There are effective solutions from Citrix and Vmware that provide extensive traditional desktop functionality to tablets.
As for the Microsoft Surface, it looks like a killer product. I'm interested, but it depends on how it is executed. 4g connectivity and a high resolution screen will be precursors for me to consider it at all.