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/dark is a vfat partition mounted with mount -t vfat /dev/hda1 /dark exportfs host:/dark results in host:/dark: Invalid argument thus making it impossible to nfs-mount a vfat partition from a Seawolf machine. I nfs-mount vfat partitions all the time as this is how I back up the Windows partitions on my machines (which ordinarily run Linux). I think this worked in Fisher and Wolverine, but I can't be entirely sure because all my wolverine machines are now running seawolf. I know it worked under 6.2. Note that exports on my ext2 partitions work fine, so this is not a general exporting problem. It is specifically that my vfat partitions can't be exported. strace shows nfsservctl returns -1 with errno = EINVAL. I haven't dug deeper than that.
NFS exporting of VFAT cannot work, as VFAT doesn't match the requirements NFS sets on a filesystem. Older 2.2 kernels didn't prohibit this and even tried to fake some of the required semantics, causing all sorts of problems when people tried to mount this from non-linux machines.
Alternatives for NFS in this case are using RSYNC or the CODA filesystem: http://www.coda.cs.cmu.edu/
These explanations are most helpful. This explains some of what I saw when I was looking in the kernel nfs server code. I guess those checks weren't there before. In my case, I'm using home-written backup software. This just gives me the push I've needed to add real remote support (a few lines of code the way it's structured -- I'll just run through ssh). Thanks for explaining the reasons for this change in behavior, which seems to be the right thing.
I'll close this as "NOTABUG" then. Feel free to reopen if you object to that.