Besides being mobile, why do people use cell phones so much at home and at the office? Some of it is clearly to save on LD charges. Some of it is that incoming may be going to the cell instead of the landline. But this says Opportunity to me. One number. Shared address books. Unified voicemail. Some Voice 3.0 apps try to get this right - like Grand Central and others - but the one that integrates that address book from the cell phone, Thunderbird, Outlook, the PDA, Plaxo, and the laptop will win.