I can now report that my system boots with an XFS root partition. I'm
posting this here in the hope that it might save someone some time and
frustration in the future. The fix was to add the following options to my
.config, from File Systems --> Partition Types
The strange thing is that this only seems to be necessary when root is
formatted as an XFS partition. If I format it, e.g., as ext3, then I can
boot with a 2.6.4 kernel that does not have these two options set. With
2.6.3, they don't need to be set.
"The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts."
-- Bertrand Russell