It's been only a month since the 10.5.3 update hit, but 10.5.4 is here now to fix a handful of issues you might have. It's a "stability, compatibility and security" update, which also improves camera RAW image support, VPN connectivity, AirPort reliability issues with 5Hz networks (oooh, we might have experienced this), and fixes some iCal bugs, some Safari bugs, and some Spaces/Expose bugs. Hit up the full list here, then download the update with your Software Update. [Security stuff]