Wireless connectivity on the Apple tablet will be important for the device to succeed.
Probably not crucial though. I wouldn't consider switching to AT&T just to get an iPhone because in my opinion Verizon Wireless has superior connectivity - at least in the Northeastern part of the United States. But obviously, other people want an iPhone enough to use one anyway.
Regardless of which network has better connectivity, Apple is reportedly in talks with both Verizon Wireless and AT&T Wireless. I will be surprised if they go with both carriers from the outset.