Are onboard accelerometers good enough to do dead reckoning positioning of the device within the building, provided they have good data to work from?

Sort of. Dead reckoning with accelerometers is only as good as the error correction. Accelerometers tend to gain error factor very quickly without using a form of sensor fusion such as a partnership with a magnetometer to cross reference things like yaw with.

"indoor gps" is a point of intense interest right now it seems like for in-shop marketing and other (evil) things.