iTunes stores all content, including video, in the iTunes Music folder.
Your movies would be stored on the FW drive with the music.
I would like to see an option in iTunes to allow you to split that into multiple locations, or keep it the same if you like.
I've always thought that video content should be in the "Movies" folder, anyway.
I guess there are some benefits to keeping it all in the same place, too.
I'm currently using a 500GB FW drive for my iTunes Music folder.
118GB is Music, Podcasts, and Audiobooks.
187GB is Movies and TV Shows.
The video adds up quickly whereas the audio slowly but surely increases.
I could get away with a 200GB drive for Music for quite some time to come, but I could see Video filling up the 500GB by itself in the near future.
Others results may vary, of course.
I guess you could uncheck "Copy files to iTunes Music folder when adding to library", but that could get really messy.