Not for a CDMA-only phone
Unless the call quality/drop issues become unbearable (and they are not where I live/work), I wouldn't switch back to Verizon (having just switched to AT&T to buy my first iPhone). Unless the Verizon iPhone has GSM compatibility (like the Verizon Blackberry's that operate on both systems) and unless the rate plans are cheaper, I won't switch back. Even though Verizon offers an unlimited Blackberry data plan, I would have to spend $20 on top of the amount I pay for 2 gigs of data with AT&T before I match it. So far, happy, so long as I can get a decent case soon!