According to the Federal Communications Commission FCC 05-116 Section III. DISCUSSION A. Scope 24.
The E911 rules the commission adopts today apply to those VoIP services that can be used to receive telephone calls that originate on the PSTN and can be used to terminate calls to the PSTN - "interconnected VoIP services."
So, if a VoIP provider has a service for businesses that meets these criteria, then the mandate would apply to them.
There are proposed solutions for meeting the E911 requirements when using Wi-Fi phones. One company addressing this is Skyhook Wireless.
You cannot have a VoIP user opt-out.
According to the Federal Communications Commission FCC 05-116 Section III. DISCUSSION C. Requirements
47. Customer Requirements. In light of the recent incidents involving problems with 911 access from interconnected VoIP services,149 it is clear that not all providers of interconnected VoIP are including E911 as a standard feature of their services.150 We find that allowing customers of interconnected VoIP providers to opt-in to or, for that matter, opt-out of E911 service is fundamentally inconsistent with our obligation to "encourage and support efforts by States to deploy comprehensive end-to-end emergency communications infrastructure and programs." Thus, interconnected VoIP providers must, as a condition of providing that service to a consumer, provide that consumer with E911 service as outlined in the requirements above.
This was first published in August 2005