Rumors have been circulating since around the iPhone's release that it would be available from Verizon in a short time. The latest rumors predict the iPhone coming to Verizon in 2009; however, that is unlikely. Apple and AT&T have an exclusivity contract that isn't expected to expire until 2010 at the earliest according to reliable reports. The contract benefits both companies in various ways. AT&T activated 4.3 million iPhones in the second half of 2008; 40% of those were by new customers. Although it is suspected that AT&T is paying up to a $425 subsidy per iPhone sold to a customer at $199 with a 2 year contract, the revenue generate from iPhone users is very high. Apple benefits because AT&T agreed it would not receive any revenue from the applications and music Apple sells for the iPhone.
So, if you want an iPhone in the near future, it looks like you will have to go to AT&T to get one.