ESMITH512  I just spent about half an hour enjoying Apple Security Update 2009-001 on several of my Macs. (I always put on my Mac Exorcist hat when I update this stuff anymore.) The stand-alone iBook updates were uneventfully routine, but a G4 Mac mini I use for network and storage services stalled, rebooted a couple times, and commanded the UPS to shut down the system after a very long screen-off delay. After reboot, the ethernet port (en0) shut itself off several times while attempting to connect to the system's DSL modem and was confusing the ISP when the ethernet port (en0) was active. I verified the DSL modem with a Linux notebook, verified the DSL modem, router, and ISP--all worked flawlessly under Linux. All systems are running MacOS 10.4.11 (networks are better on Tiger) and always have been kept up to date. On the Mac mini G4 network/file server, I went to Mac OS's network configuration assistant from the network control panel and re-entered the DSL login information, after which the system resumed normal operation without further incident.  
(Version 2009-001)