Looks like your module-init-tools is either outdated or damaged. Do the modprobe and insmod binaries in your $PATH match the ones installed by module-init-tools?
$ which insmod modprobe /sbin/insmod /sbin/modprobe
$ pacman -Q module-init-tools module-init-tools 3.10-1
Surprisingly, inserting the module with insmod had worked!
$ insmod /lib/modules/2.6.30-ARCH/kernel/fs/isofs/isofs.ko $
And the DVD mounted just fine. Modprobe, however, still throws the same error. I tried to make it more verbose:
$modprobe -v isofs insmod /lib/modules/2.6.30-ARCH/kernel/lib/zlib_inflate/zlib_inflate.ko WARNING: Error inserting zlib_inflate (/lib/modules/2.6.30-ARCH/kernel/lib/zlib_inflate/zlib_inflate.ko): Invalid module format FATAL: Error inserting isofs (/lib/modules/2.6.30-ARCH/kernel/fs/isofs/isofs.ko): Invalid module format
Check /etc/modprobe.d/ for isofs or zlib_* etc. Also try running "depmod -ae", and the modprobe ... -- damjan