From the investigation performed, it looks like the database has been compromised through a specific site script (not related to the forums - the security level of the forum software we use is known to be pretty high. In any case, I've applied the latest security patches to the forums).
Security is taken very seriously; we have had many measures on the server to prevent things like SQL injections and so on. But unfortunately, it looks like a loophole was found. It has been patched now.
In the next month or so, we'll update the forums software to ensure we have the latest security features (as well as the latest cool forum features).
Our sincere apologies to everyone for this mishap. Rest assured however that the only sensitive information we have in the database are the emails. The passwords are stored in encrypted form, so they are perfectly safe.
Hopefully, this will not happen again.