I actually had to split the storage. My Time Capsule is pretty much my main iTunes repository. I have a Mac Mini that controls the music management. I used to manually sort my music by preference/genre. Then one day I accidentally had iTunes organize my iTunes library and I couldn't go back.
Yeah, happens to a lot of people... I have iTunes store/organize all music on the Time Capsule. Then my wife uses her Macbook to sync her iPod and I use my Macbook to sync my iPhone 3G. To save HDD space on our Macbooks, we (and I mean me) point our iTunes library to the Time Capsule but don't copy them locally. Yeah, we have to be 'home' to sync but I rarely sync out of the house anyways. My iTunes library is relatively big, especially now since I've committed to Apple lossless and Time Machine eats into the Time Capsule's 1GB So only Time Machine and iTunes are on the Time Capsule. For all other stuff, like random files, digital pix, home vids, movies, etc, I put them on the LaCie on the other Airport Extreme.
Now I haven't tried this but you can connect a USB hub to the Extreme and then add multiple HDDs. So if you have a bunch lying around (like I do) they can all be made into NAS's. You can use the Airport Utility to control access to each of the drives.
And to make it more complicated
I use Amazon's S3 and Jungledisk for off site storage. My previous LaCie died last year, taking many years of photos with it. It's beyond repair, unless I spend $$$. After that, I now use Jungledisk to save my photos and home vids to Amazon on a set cycle. For about 13GB, I pay about $4/mo. Great for piece of mind in case something happens at my house. I don't do it for my backups or my iTunes. I figure I can rerip if necessary and I'm too cheap to store backups. Granted it helps that my Comcast cable speed is 10Mbps upstream.
