When my son had a febrile seizure last year due to a high fever (an apparently common but totally scary experience), my wife called 911 using the landline. The ambulance was here in less than 90 seconds.
The thing about 911 is that on a landline, at least where I live, you are directed towards your local 911 dispatcher (often the police station), and as soon as you call they have your address. With a cellular phone they can triangulate and connect you to the highway patrol or county dispatch router (they don't have your address), and VOIP has their own homegrown solution, but these methods can add minutes until help arrives.
There are some claims that 911 will still work if you shut off your service (though the phone company couldn't make that verbal commitment to me), or that mobile phones can pinpoint your location, but there's no guarantee. In fact, the wireless carriers are being FINED for not meeting stricter requirements for locating emergency callers.
More articles:
Have you dropped your landline yet? CNet
Net Phone Zone: The 411 on 911: PC World
Technology: Cutting the (Phone) Cord: MSNBC
Tags: phone