I recently ran into this as well. At first I thought the problem was my router, which is hosting the NFS, and it was partially as it changed the paths for some reason (/tmp/mnt/<volume> -> /mnt/<volume>), but changing the paths didn't help. I also tried downgrading the kernel as someone said here, but it didn't help either... The Pi boots but then it starts printing "nfs server 192.168.0.1 not responding, still trying" and "nfs server OK" in between the output. Other machines in the network, including another raspberry pi running directly from SD, is able to mount the nfs's with the same settings... It also fails to mount the boot partition (probably because root fails to mount in the first place...)
EDIT:
Messed around with my router a bit, and now the shares are back to having /tmp/. I also added the 'nfsv3' (found some other arch linux forum post where someone said it changed from nfs -> nfsv3) module to mkinitcpio.conf and generated a new initramfs, so let's see if that solves my problem. I probably won't get a chance to try until tomorrow though, will report back.
EDIT2:
Yep, the problem was with my router after all. Or actually, the hard drive connected to it. The journal files had broken somehow, causing input/output errors and I couldn't delete them or do anything else with them either. So I plugged the HD into my laptop and ran fsck and then deleted the old journals (there was 27gb of it
) and now my nfs root works again (with the latest kernel etc.). Woohoo !