I'm using the XeTeX version that ships with TeX Live 2011. A recent update from zlib form 1.2.5 to 1.2.6 broke it. When running xetex against a (possibly empty) tex file, all I get is:
This is XeTeX, Version 3.1415926-2.3-0.9997.5 (TeX Live 2011)
restricted \write18 enabled.
(Fatal format file error; I'm stymied)
Regenerating all format files using fmtutil-sys --all runs successfully, but the error doesn't vanish.
Here are some distro bugs about this issue:
Reverting back to an older zlib release (1.2.5) fixes the issue.