Tried to replace the battery in my Sonicare with no success. The thing was not only built to be thrown away, but built so that you'd never be able to replace the battery without some serious professional soldering work.
A solution for those who are in the market is the iBrush. I just bought it and found it comes apart easily just using a philips and flathead screwdriver. Once you've disassembled it, the battery is easily accessible (and I presume replaceable, although I won't get to test this until 3-5 years from now).
As far as I'm aware, this is the only ultrasonic toothbrush on the market that allows for the possibility of battery replacement without major soldering efforts. On top of this, the brushing quality is tremendous, probably a bit better than my old Sonicare, and certainly better than the Oral-B model I've tried.
good green thought +
Second that + Ralph