I also have an ibook G4 1.33 12inch with a 1.5mb, I just installed a new HDD 60GB/5400 and did a fresh install of Leopard 10.5.1, it has transparency just fine, did you do a clean install or upgrade?
Video is same as yours so it should work...
On the other hand did a fresh install on a iMac 800 15in flat panel and I did not get any transparency on the top bar (but normal menus has some transparency) and I read that someone else did have it on the top menu... go figure... I think its a bug on leopard...