The Digitial Rights Ireland site has a nice introductory post regarding data retention.

Reporting of events in the EU is sparse in Irish and British newspapers. So, to try to keep our readers up to date, we’ve compiled the story of the push to bring in data retention across the EU, up to now. It should serve as a useful mixture of background information, jargon explanation and opinion collation.

They’ve also put up a good sample letter to send to your MEP on the issue.