Well, despite the context that VoIP is a different service than telephone service, the FCC orders E911 compliance.
The Federal Communications Commission dials into E911 for voice over IP service providers. In a decision released on Thursday, May 19, 2005, the commission identified that fundamentally substitute services that connect with the PSTN must allow consumers to call 911 and get the emergency service they expect.
Not to be confused with IM-type, PC-to-PC calls in games etc, only those services that connect to the PSTN are required to comply within 120 days. The order also specifies incumbent carriers to facilitate connection (some haven't been doing this).
This decision is actually a good one. Simple and clear, it addresses the consumer expectation for 911 connectivity for a substitute service like VoIP, and overcomes the objections of the legal beagles working for the nation's carriers.
Enterprise-wide deployments, on the other hand, need to develop a strategy for exactly this requirement.