Just a followup to my last comment. VoIP providers could ask one simple question to determine whether a customer qualifies for full 911 service:

1) ___yes ___no - Do you plan to use your VoIP phone solely in your permanent residence?

If the answer is "yes," you'd qualify for e911 service. If your answer is "no," you'd be given three options:

1) Sign an agreement stating you understand that 911 calling will not work if you go mobile ... or move to another permanent residence without informing your new provider (and getting a new number if your area code changes).

2) Get VoIP service without 911.

3) Get a cellphone.

It's really a no-brainer.