Can't shutdown because an NFS share isn't being unmounted

I have a particular NFS share that apparently is buggy. Once mounted, it can't be unmounted, always reporting that it's busy. Consequently, the system hangs toward the end of the shutdown process (at the partition unmounting step, I believe). It's then necessary to use the Magic SysRq functions to reboot in a somewhat-nice manner.

Here are the details. I'm experiencing the issue on a clean install of Feisty, but I previously experienced it in Edgy (upgraded from Dapper). My server is still running Edgy--also upgraded from Dapper. I have four NFS shares in my fstab. Only one gives me problems. I suspect that something's wrong somehow with it, but I don't know what to do to troubleshoot it. One of my shares exports a subdirectory of the problem partition; that share causes no problems.

I can work around the problem by writing a sysvinit kill script that unmounts all partitions mounted on the troublesome mount, then remounts the buggy share read-only. I run this script before any NFS or network kill scripts (K02). When that happens, the partition gets unmounted properly and doesn't hang the system.

I'm really not too sure about in which package this bug should be addressed. I listed sysvinit because ultimately, there should be some sort of timeout (at the minimum) so that a misbehaving program can't hang the system. On the other hand, this might be an issue with NFS.