When I look at an individual post using Mobile Safari (on iPhone 4S, iOS 5.1.1) there is a <-- Previous or a Next --> link, or both. Those links do not activate when I touch them. The post date is also shown, and that link is active.
Looking at the site in a Desktop browser (like Firefox) shows the Next and Previous links to be active and working. So it's just when viewing on my Phone that they are broken.
I'm using the twentyeleven theme. Some customization, but only the font properties of h1/h2/h3 within the entry content.