Assuming that you never make calls on the road, maybe you can get rid of your pesky cellphone plan altogether--and use your Wi-Fi-only tablet to make phone calls through a service such as WiCall (Android), which lets you call over your wireless connection.
These calls aren't free--they cost $0.009 per minute for U.S. calls--but that's a pretty negligible charge. If you'd prefer to pay up front, you can sign up for a service such as GrooVe IP (Android, $5), which lets you use your Google Voice account to make free phone calls.
iPad users can check out the free MagicJack, which allows you to make free phone calls to any U.S. or Canadian phone number.
Data Plans--Who Needs 'Em?
Considering all of the things that a tablet can do without a data connection, it's no wonder that people are overwhelmingly choosing to purchase Wi-Fi-only models. 3G/4G tablets cost more, they require an extra data plan, and they will incur an extra charge on the new shared data plans.
In contrast, you can do just about everything you need to do (except, perhaps, answer emails on the fly or make phone calls on the road) with a Wi-Fi-only tablet. And if you ever do need an Internet connection, you can tether your tablet to your phone or sniff out a wireless hotspot.
Sorry, wireless carriers: it looks like Wi-Fi-only tablets are here to stay.