At first, I was kinda concerned by this. But then I realize, my online usage with a smartphone would probably just be checking email, weather, and a few websites along with downloads from the Android Marketplace.
So lets say Verizon's highest data plan option is 5Gb. That actually gives you a lot to do.
Right now, we have Verizon's wireless keycard for our internet with a cap at 5 Gbs a month.
I mention to stretch that 5 Gb out pretty far. And I am a heavy internet user, downloading Development applications and visiting a few websites daily.
So basically, you can go far with a 5 - 10 Gb cap.
Why would anyone need 20 Gb of free transfer anyways from a smartphone?
The only way I can see someone going over 10 is with tethering and even then, you almost have to be using the internet all day for a month or download a huge program. (Photoshop, 3DSM)
There's a guy here that's eaten over 50GB in one month with his phone so it is possible to go way past 5GB (10X in fact!).
Keep in mind there are a lot of people that waste bandwidth by streaming music. It's pretty easy to go through a ton of data if you stream 8 hours a day 5 days a week or more. Figure 80kbps or 10KBps times 3600 seconds per hour times 8 hours per day times 5 days per week time 4.3 weeks per month and the grand total for music streaming alone is ... wait for it ... 5.77GB. Now do 16 hours per day times 30 days per month and you can go through 16.5GB just streaming music.
Now, add in streaming video and downloading hours of porn and you can go through huge amounts of data. Of course most of us don't and won't, but if we did there's no way the carriers could handle the load and the network would choke to death. I don't want my cell service to quit on me because others are listening to hours of pop (crap) music.
Brian