Force umount of stale NFS shares
rWhen you kill iTunes, do you kill iTunes Helper too?:

$ ps -cef | grep iTunes
1001  7878   243   0  8:30AM ??         0:05.38 iTunes
1001  7882   243   0  8:31AM ??         0:00.03 iTunesHelper


If you can't kill iTunesHelper also, you will need to add "intr" to the mount options. Having done that, you should be able to stop processes waiting on IO from the unavailable filesystem. Once you can get rid of processes using the filesystem, you should be able to umount it.