The OBi already has this capability -- see
http://www.obitalk.com/forum/index.php?topic=2319.0 . You should test that it works as intended. Get permission first, by calling your police non-emergency number. When you make the "911" test call, immediately tell the dispatcher that there is no emergency and confirm that you reached the correct center.
However, I strongly advise you to get real 911 service. It's as little as $0.80 per month from Anveo, or $1.50 from Callcentric (with a much longer track record for reliability). Real 911 can get you help if you are unable to speak (stroke, choking, etc.), don't wish to speak (intruder in house, calling surreptitiously in a robbery), or the call is made by a guest, babysitter or worker in your home who doesn't know your address.
The SIP service that you'll have for 911 will also serve as a backup to GV, be useful for determining whether a problem is provider related, and allow for interoperation with many useful other services.