Quite soon after the previous stable release, another one. We missed a few issues that Secunia had found, which we felt was better to address sooner rather than later.
The security issues that were dealt with require having login details to your phpList installation. Therefore they are not exploitable by unknown outsiders, which makes the threat a little less. Nevertheless we advice everyone to upgrade to the latest stable version. In general that's a good idea with any software you use.
To get the latest version, and upgrade, fetch your copy and do the following:
1. keep your config file
2. overwrite all of the phpList code with the new version
3. put your config file back in place
4. Go to your admin page and click the "upgrade" link.