Latest 4.7-fremantle git build (2010-06-06). Open youtube.com, select any video - there's sound, but no video. Click on another video, and suddenly there's video + sound. Note that 4.7-fremantle uses "-graphicssystem raster" by default. Overriding with "-graphicssystem native" fixes the issue, so I suppose it's a bug with native WIds with raster+maemo? Note2: This seems to be a regression against 4.6-fremantle, which used raster and youtube played just fine.
Sorry, forgot to mention that this is happening on the N900 with PR 1.2. Qt 4.6 with PR 1.2 works.
Looks like that particular version of flash was buggy. After a firmware upgrade to latest PR, Youtube works again.