It really depends on what kind of business you want to build, and in what kind of market. You shouldn't even want to be a software entrepreneur, you should want to solve a problem / improve things. Some problems can benefit more from mobile than others. For example, PCs are great for high I/O tasks like authoring and formal communication, while smartphones are great for low I/O tasks such as consuming infotainment and informal communication. Most areas involve a mixture of those, and which should be your first priority depends on your area.
For example, meetup.com is quite PC-oriented, which almost seems odd for a new social networking platform, but being able to easily publish and read detailed meetup descriptions is critical to its success. That doesn't mean meetup.com cannot benefit a lot from mobile (I wish it did), but it shouldn't be its first priority.