(Warning: Incoherent rambling below)
My husband was mentioning to me yesterday about how one of his coworkers had dropped their land line and was now using Vonage (an IP phone) exclusively. Some of my other friends have dropped their land line and only use their cell phones. I personally will never do this. I see the land line as insurance.
Insurance is why I got my cell phone in the first place. I wanted a phone to take with me (or to send with my husband) so that if something happened, I could call for help. But, I wouldn't trust my cell phone as my primary line. First, I have on occasion mislaid my cell phone, so I call it to find out where it is. I guess I'd be more careful or email my husband to call. Second, the battery life. My cell's batteries last about 2 days at best. I'd hate to need to make an emergency call and have to find the charger first. Finally, what about the babysitter? Do you leave your cell with him or her or what?
With VoIP, I'd worry about emergency situations and the internet service being down. My internet service is fairly good. It hasn't been down much at all in the several years we've had it, but I'd hate to have to trust it for a 911 call. The second issue is power. What if the power is out? In my (limited) experience with power outages, the power is more likely to be out than the phone (this of course assumes a phone that gets its power from the phone line).
I think 911 is the main sticking point. With a land line, 911 can tell where I am and send an ambulance even if I can't talk or had my 3 year old daughter dial it because I was unable to. The same can't be said for cell phones or VoIP.
Yes, there are ways around all the difficulties I've just mentioned. I'd just rather keep my land line (plus, I get free local calls :) ).