QuickTime 7.4 / Perian subtitles do not work?
You should update to the Perian 1.1. For QuickTime 7.5 there is no Perian update available yet. Also QuickTime 7.4 did break “Export as subtitled QuickTime movie” on Mac OS X 10.4 Tiger, use the newer “Export as QuickTime movie” instead. If you need multi-track subtitles an interim workaround has been prepared.