To the OP- I'm in the exact same boat. My Verizon contract is currently up on January 21st. I want to get the iPhone, but I don't want to make the 2-year plunge into ATT before I am sure I like it, and the iPhone itself.
So, here's what I'm thinking of doing...I am going to get the iPhone today or tomorrow, and get it with a new (or temporary) number.
I will tell all my close friends my "new" number, or people that text or call me often, so that when they call me I can get a feel for the iPhone and how ATT's network is. Meanwhile, I'll carry around 2 phones for a bit.
Like I said, my Verizon contract is up on the 21st. If I decide to make the commitment to ATT before that date, I'll be able to switch my Verizon number to the iPhone, and thus end my Verizon contract.
If I need more time to decide, I'll extend my Verizon service with a pre-paid plan so I can keep my number and service but still test-drive the iPhone.
If I don't like ATT, I can cancel within 30 days, pay for what I used, and continue my service with Verizon.
I have thought about: If they come out with a 32GB model and the 16GB becomes $199 within my 30 days, can I cancel everything and then go to a different ATT store and do everything over again?