Click on the battery icon at the top and click 'enery saver preferences' then untick 'dim brightness before going to sleep'. You'll need to do it for battery and power adaptor modes.
I unchecked "Automatically reduce screen brightness before sleep" once and my 15" MBP running Leopard never does it (well actually twice - once for battery and once for power adaptor).
I use a small program (~50kb) called Caffeine to prevent it from dimming. Runs quietly in the menu bar. Of course this doesn't solve the underlying issue, but it's great for watching web videos.