Apple has released QuickTime 7.4.1 which…
…addresses security issues and improves compatibility with third-party applications.
I installed the update for Leopard on my iMac, MacBook and Mac Mini (all Intel based) without a problem. Like QuickTime updates that came before, a restart is required.
There were reports of problems with the 7.4 update and video playback although I didn’t have any problems. This update does not appear to resolve the issues for those who had the problem and some reported the problem with the install of 7.4.1 even though 7.4 didn’t have cause a problem. MacFixit has information and possible resolutions.