In FIOS areas, it's no longer possible to get a POTS landline. You can get a phone service over FIOS, but it's subject to wall-power being available, and you're using the same E-911 system as normal VoIP or cell phone services, anyhow. It's the FCC that's to blame for me not having a landline.
Also, there's no reason cellular 911 service shouldn't be ultra-reliable. There are 4 different nationwide carriers in the US. What are the odds that all 4 of them will have ALL their overlapping cell towers in an area knocked-out? That does happen, today, but ONLY because the FCC pussied-out on requiring them to have backup generators in each cell tower, and lets them just keep a few backup batteries in there for short power outages.
And if some event damages the fiber-optic line to my house, there's no chance I'm fixing it... At least with a cell phone I have the option of climbing onto higher-ground and trying to get a signal from a more remote tower, or even just SMS texting emergency services (coming real-soon-now) and hoping.
With ad-hoc WiFi in cell phones, people may soon be able to self-assemble into their own wireless network that spans whole cities, after a disaster knocks-out all other local service. Try that with your land-line.