The Serendipity Team is still working hard on finding the time to not only work on new features of the 1.1 version of Serendipity (like a new per-template configuration options, a completely redone media gallery and media permissions), but also to finalize the long overdue 1.0 release. We are getting close on settling up the design debate, so stay tuned for an update on this.
It's our goal to have the 1.0 release finally be done this month; the codebase is rock-solid, and the 1.0-beta3 version should be ready for showtime for everyone; there are no open bugs or issues known to us, and this version is to prefer over the old 0.9.1 version currently.
If you are already using 1.0-beta2, there is little reason for you to upgrade. beta3 mainly includes some minor bugfixes as well as some last-minute features to force FeedBurner feed redirection, the most recent updates to the Spamblock plugin, a Pivot importer, missing UTF-8 encoding for Windows-Server date responses. Also two minor security issues were addressed. One being an (hard to do) XSRF-attack on the entry manager page, the other one being able to save PHP code in the serendipity_config_local.inc.php file. Only an Admin can do that, and usually and Admin has other means to save PHP code on your server ;-).
Since those changes remedy a new release candidate before the final 1.0 release, we would be happy for anyone trying out this version and report us possible new issues. beta2 has been very well received so far, and our top priority for the 1.0 release is to have a rock-solid release. Without you fellow users, we can't find all bugs, so it's also up to you to help us. Drop by on the forums to report bugs you found!