Hardwired Smoke Detector
I replaced mine in 2011. The new one is slightly smaller than the old one but is big enough to cover the whole in the ceiling. It also looks better than the old one. They are very cheap at $12.81 & free shipping (February 2014 price): Hardwire Smoke Detector with Battery Backup
You can easily hook it up yourself if you use twist-on wire connectors.
I like wing-nut type. Color indicates the wire sizes it can accommodate. These connectors are for copper wires only (we have the aluminum ones) but the amount of current used by the smoke detector is so small that you can ignore it.
NOTE: Because of the recent law changes, i you live in a unit with a fireplace, you need a dual unit, a smoke detector/carbon monoxide detector. Some examples: Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup