Whether a mobile site is necessary really depends on the type of industry.
Many websites need to be accessible to people when they are on the move as well as when they are in front of a computer. But many smart phones today can go to full pages without any problems.
I have a friend who has built a daily deal website, EF Buys. It is doing Deals for Small Business. He said he did not need any mobile accessible sites. Because deals for small business are not something needs to be done quickly while people are on the move.
But groupon, on the other hand, is a daily deal site for consumers. They do need mobile sites. I also recently heard that they were going to launch a push-button at every cash register in super markets.