I'm having a similar problem installing slackware 11 to my laptop with a broken DVD drive, I have successfully booted from PXE (using the huge26 kernel) intending to install from an NFS share. At first it was just kicking me back to setup's main menu after failing to mount the nfs, but once I ran 'network' and specified my nic's chipset it would give me second and third chances to mount it.
I've had my NFS server working properly for months now and am able to browse the share once it's mounted on the laptop. I had failed to set the correct permissions, but after fixing that nothing changed.
In the laptop's /var/log/messages it says "auth.err portmap [some port]: cannot bind udp: address already in use", I tried reloading my nfs server's exports and restarting the nfs server but that didn't help either. I also restarted the portmap daemon on the server to no avail (even rebooted the whole thing after neither of those worked). I googled with that error message but only got one hit which wasn't exactly relevant. I did find a site called
slackbasics dot org (I understand that people can't post hyperlinks until they've posted thrice to avoid spam... and I don't feel like posting trivialities so it'll be a couple more posts before I post a link
which suggested I manually start rpc.portmap on the laptop but that didn't help either.
One weird thing I noticed is that the setup program does not umount the nfs shares which failed, so I have been manually unmounting them in case the setup program was confused by two different nfs shares mounted on the /var/log/mount dir. Is this a waste of time?
I downloaded the dvd-iso from the slack 11 torrent and have installed from the iso previously so I'm sure it's not corrupted. I installed Fedora from nfs a couple of months ago and it was way easier than this - of course, with Fedora you just point to an nfs share with the unmounted iso.. Too bad I hate fedora once it's installed
..