Olaf, can you please check if my patch for busy inodes from -mm tree helps you?
Patch name is fix-of-dcache-race-leading-to-busy-inodes-on-umount.patch
http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.15/2.6.15-mm4/broken-out/fix-of-dcache-race-leading-to-busy-inodes-on-umount.patch
This patch is just wrong. It is hiding bugs in file systems. The problem is
that somewhere the reference counting on the vfsmount objects is wrong. The
file system is unmounted before the last dentry is dereferenced. Either you
didn't hold a reference to the proper vfsmount objects at all or you
dereference it too early. See Al Viros patch series (search for "namei fixes")
on how to fix this issues.