What sort of power outage spans are we talking about here? A few minutes? A few hours? A few days? Do you know if internet services stay active during a power outage? (It
should for the same reasons why the landline stays active,
especially if you're using DSL. The way to confirm on your end is to have your modem connected to a UPS during an outage and see if you can get online. If you can, you're golden. If not, it's because the clods who designed your network didn't bother throwing batteries into the HFCs or MSANs.)
The thing to understand with VoIP and e911 is if the local PSAP supports e911, you
should be fine. That requires a bit of research on your end, however. You can normally set up a test call with your local 911 PSAP, contact the local police department and they'll tell you how. Contact any prospective VoIP providers and see which PSAP they route to for your area as well (check with VOIP.ms first). If you can maintain internet access during an outage and the local PSAP can handle e911? Buy a decent UPS, an ATA, set up an account, and stick your modem, router and ATA on the UPS.
There's a whole section on 911 services via VoIP here (as well as a whole slew of info on frugal, quality VoIP providers):
http://www.techmeshugana.com/theguide/voip-providers/Alternately, if you've got good AT&T wireless coverage in your area, you could probably pick up a cheap used AT&T Wireless Home Phone device like the ZTE WF720 for a song off Ebay and stuff an Airvoice or other AT&T MVNO SIM card in the thing (it should work, AT&T carrier locked devices will still do calls and SMS texts with AT&T MVNO SIM cards - carrier unlocking is only for data services and MMS on the device which this would need none of, or for taking it to a T-Mobile based provider). These things have built-in battery backups, too. The only downside is that the e911 service location is going to be based on wireless triangulation, so 100m radius unless the person calling is coherent enough to actually give an address.