Sorry for the delay. I tried to figure out what could cause your installation to fail and didn't find anything. 188.8.131.52 is out, but chances are that it will behave the same, since it's the same installer. It's not too different from the original Debian Installer, but it's not the final release. It's a pre-RC3. If Debian Sarge works for you and Pluto doesn't, let me know and I'll upgrade our installer too.
The integrity check fails because that file is generated by mkisofs it seems, which is one step after the md5 sums are calculated. I removed the isolinux directory from the md5sums file today.