1. Create a stand alone mobile website
2. Use "responsive" design to ensure that your website renders well on small screen devices.
3. Create an iPhone/Droid app
4. Do nothing and hope mobile browsing was only a fad.
The best strategy for your particular situation depends a variety of variables including:
1. Site complexity. More complex sites are harder to move toward responsive design.
3. Mobile traffic
5. "Must have" features
While responsive design is the current hot topic in web dev I've found that for clients with highly complex "must have's" (and large enough budgets) native phone apps and stand alone mobile sites are still the way to go.
http://www.howtogomo.com/en/d/ - and then using this as a test case for your mobile efforts.
Go|Mo allows you to quickly create a mobile friendly version of your site (or any site!) - interact with it - share with friends and customers - and really get a sense of how your existing content could be formatted for mobile. The mobile version of your site won't be as professional and well crafted as it would be starting from scratch with a good developer - but in a matter of minutes you'll get a sense of how your optimized site could look.
As a test case I decided to create a mobile optimized version of noaa.gov (my favorite weather site). Their website is complex and poorly designed (for both traditional and mobile visitors) - and gets even worse when viewed from an iPhone. In spite of the poor design I still favor this site because of the lack of ads and the quality of data. The solution? Use GoMo and create an awesome mobile friendly version of my favorite site.
Getting started required simply copying the URL of my forecast page:
http://forecast.weather.gov/MapClick.php?zoneid=VTZ017 and then entering this into the GoMo home page.
The GoMo tool allows you to choose from a variety of "skins" and also allows you to modify the layout (and even included content) by adding or removing blocks of content from the mobile version of your site. The actual "mobile friendly" version ends up hosted on duda mobile - this is the URL for my GoMo forecast site (Chittenden Vermont only):
While I'm not recommending that anyone use GoMo to create the final mobile friendly version of their site - it is a good way to see the possibilities -and also garner feedback from friends and customers. It also allowed me to create a mobile friendly version of one of my favorite sites with very little effort.