This measure is welcome, but it only goes half way to solving the problem of huge video files inundating a Mac's HDD.
The balance is tipping well in favour of people owning laptops these days rather than desktops. Most laptops don't come with more than 250GB unless you upgrade it yourself. Many still come with 120GB!
With that in mind, most people would rather not have all their video files reside on their laptop's HDD but on an external disk.
I have my 350GB iTunes library on an external HD, with only music (30GB out of that 350GB) on my laptop. Granted, it's not quite what you want, but it's working quite well.
I moved my library onto a 1.5TB external drive, then removed everything but music via Finder (NOT iTunes!) from my laptop (this bit would have been so much easier with iTunes 9). I set the iTunes library location to the external drive, and checked that my content was available (randomly checking Get Info on a few items). ... edit: actually, I'm a bit hazy on this bit. It's been some time, but I think I'm right.
When I quit iTunes, disconnect the external drive, and restart iTunes, I now get the famous ! by all my video files, but all the music is accessed from the standard iTunes location in my home folder on the internal. Restarting iTunes with the external drive connected now grabs all content, including Music, from the external.
When I download music while the external is disconnected, it goes onto the internal drive, but I select "Consolidate" to move it onto the external drive when I reconnect.
Like I say, it's not quite what you want. There's a copy of the music files on the laptop, I have to remember to quit iTunes before I leave home or when I get home and want to access video, and I have to make sure I add music only when my external is disconnected (so I don't have to worry about getting a copy of new music onto the internal drive)