Probably some incompatibility between cryptsetup 1.0.7 and kernel 2.6.31.x or just a bug in 1.0.7
I can't really know but here's the changelog anyay
http://code.google.com/p/cryptsetup/source/browse/trunk/ChangeLog
In any case, the ArchLinux cryptsetup package maintainer said he will email the upstream cryptsetup developer.