The truth is that most businesses probably don't need a mobile version of their site yet, but if you are tempted to build one, then I really hope you have a series of other, more fundamental, bases covered already.
Throughout the evolution of the internet and web marketing, there have always been cutting (bleeding) edge applications that have garnered a lot of attention. Early adopters both in terms of vendors and users were clearly ahead of the pack, but they also paid a significant price for their front running status. Sometimes that path proved to be worthwhile and other times it didn't. Examples of past emerging channels include video, local search and social media. Each went through a tremendous number of iterations over several years and their early versions were often less than effective. Over time, they evolved and are no longer on the fringes, but it's reasonable to question whether pursuing them (at the expense of other more established marketing channels) was the right move for companies in the early days of their release.
I don't think anyone would argue that video, local search and social media aren't mainstream building blocks of internet marketing today. For instance, it is becoming imperative to have a social media strategy; if for no other reason than to remain competitive from an SEO perspective, as these listings are increasingly visible in the natural results. Facebook, LinkedIn and the rest have tremendous traction and they warrant an allocation of resources at this point.
I remember overhearing a conversation a couple of years ago between a peer in the SEO/SEM industry and his client. The client, a major brand name manufacturer, was calling, outraged that his ad was showing up a on MySpace page that had a lot of racy content. There wasn't much that my friend could do short of pulling the campaign altogether, as MySpace didn't have a full scale ability to filter the pages that ads appeared on. Today, there is greater potential for targeting.
Similarly, if a business has a physical presence and is looking to attract foot traffic, then local search ought to be a central part of the game plan today. That being said, there are still a tremendous number of errors in local directory information on all of the major engines, yet accuracy-wise local search is light years ahead of where it was a year ago.