Apple normally includes the latest Flash Player in their OS X updates, but 10.5.3
still doesn't include Flash 9.0.124.0, which Adobe released in early April. This is really irresponsible of Apple, given that 20,000 or more web pages out there have reportedly been compromised to include code to redirect to sites exploiting one of the (many) vulnerabilities in 9.0.115.0:
http://blogs.zdnet.com/security/?p=1189
http://blogs.adobe.com/psirt/2008/05/potential_flash_player_issue_u_1.html
Granted, those exploits are
probably Windows-only, but now that Macs use the same processor architecture as Windows boxes, Apple can't afford to just assume no one's going to go to the (relatively small) extra trouble to make variants of their Flash (and other cross-platform software) exploits for OS X.
Anyhow, OS X users who don't want to be vulnerable any longer will need to go to
http://www.adobe.com/go/EN_US-H-GET-FLASH to install 9.0.124.0 using Adobe's installer.