ScottD tipped us to the LiPo Auto Balancer circuit featured in the Elektor magazine’s June 2010 issue.
When using multiple lithium ion/polymer batteries in series an auto-balancing circuit is needed to make sure all the cells charge equally. Due to slight differences in capacity, some cells will reach full charge before others. This effect increases after each charge/discharge cycle, and could lead to premature aging of a cell or even total breakdown.
The circuit provided in the article balances the charging of up to 4 cells connected in series. It compares the actual potential of a cell to a voltage divider that provides the average cell voltage. If a cell potential is greater then the average voltage the circuit starts discharging it, and if the potential is lower the circuit increases the charge rate of the particular cell.
Article is free but registration is required, also provided is the PCB layout PDF.
Via the comments.