Ulf Wendel is presenting several good articles about the process about implementing the mysqlnd library for PDO as PDO_MYSQLND. I wrote about PDO_MYSQLND when it first was announced, but Ulf has posted two good articles about the implementation of PDO_MYSQLND since then. These articles provide a unique insight into how PDO is built and what challenges lies ahead for PDO2 (.. in addition to the license and NDA debate..)
Remember that you can always follow the latest developments over at Planet PHP.