I'm very excited about the future of VoIP - unfortunately there have not been many compelling reasons for most businesses to adopt the technology. To date, primary motivation comes from cost savings when you can use VoIP systems for toll bypass. Because of this, VoIP systems are adopted by larg(er) companies with multiple locations that can show a cost savings and those who like to spend a lot of money on cool new gadgets.
VoIP systems will become more prevalent once there is a "killer app" out there that makes these systems a must have. Integration between your computer and telephone is going to encourage businesses of all sizes to take a serious look at VoIP.
Now that Cisco has finally gotten on board with SIP (Session Integration Protocol), VoIP adoption should become more worthwhile for a single location business. I point to two main reasons: SIP will make VoIP systems more intelligent, bringing convergance closer to realization; and secondly, SIP systems will be more interoperable.
By being able to combine a product such as Cisco Call Manager with lower priced phones from another vendor, there will be more pricing pressure put on the industry. While I find the Cisco VoIP system to be the most interesting, you're shelling out a lot of cash vs. a traditional POTS.