A recent Wall Street Journal article nicely summarizes statements large fleets have made regarding their plans to purchase natural gas trucks.  Specific figures include:

FedEx: 30% of trucks will be NG over the next decade
Lowe’s: 100% by 2017
NFI Intermodal: 30% within five years
Procter & Gamble: 20% within two years
Ryder: 10-20% within five years
Keep in mind these are just highway vans, trucks, and tractors. Municipal trucks such as garbage trucks are already largely natural gas-powered. 
The usual factors are mentioned as unknowns – fueling infrastructure, fueling procedures, fuel economy, cost. At the same time, the numbers these fleets discussed are somewhat eye-opening. Most analysts predict 5% of the nation’s fleet will run on natural gas in 2014, up from 1% this year. That alone is a substantial increase. Analysts may have to increase their estimates going forward if the above numbers hold true. 
As for used natural gas trucks, there is essentially no sales data available. Until very recently, the limited engine choices combined with the lack of a fueling infrastructure kept natural gas power an extremely small niche. I assume finance entities are calculating residuals based on internal estimates and guidelines. We expect to see a secondary market start to develop over the next three years, and will make valuation judgments as soon as a usable volume of data is collected.