Habari is "in" now, I suppose. I've been following (but not participating in) its development, and it looks extremely promising. Khaled and Chris are both part of the team, as well as a few others. Once an early beta is released, I'm hopping on the bandwagon and switching over from WordPress.
Habari relies on PHP5 with PHP Data Objects (PDO), and your choice of SQL database (MySQL, PostgreSQL, SQLite). Habari is strongly object oriented, and implements the full suite of the Atom Publishing Protocol. User-created plugins make Habari do nearly anything imaginable, and a robust theme system permits the use of several popular templating solutions.