Over the weekend, the mailbag caught a few notes of problems starting up once Leopard users had applied the 2009-001 Security Update (thanks @danielbru), and on Friday the spotty issue was noted by MacFixIt as well. Affected users report a variety of symptoms: the dreaded 'boot disk not found,' a 20-minute hold before the login screen shows up, and a disabled keyboard & mouse -- making the usual first troubleshooting step (a Safe Boot, triggered by holding down the shift key at start up) rather more difficult. As with most Mac OS X patches, the majority of those who applied the security update did so with no reported problems, but that's cold comfort if you're one of the unlucky few.
While it's always a good idea to verify your backups before updating to major dot releases, I doubt most users would bother with that due diligence when facing a 'simple' security update; unfortunately, since these updates often touch multiple subsystems, issues do emerge for some folks in the aftermath. MacFixit's thorough guide to troubleshooting startup problems is a good starting point if you're having problems, and if you've already been through the mill this weekend, let us know how you resolved your problems in the comments below.