dpkg 1.16.0 has entered Debian/unstable yesterday, and contains a change which currently prevents installation of Grml kernel packages. Quoting the changelog from dpkg version 1.16.0:
Do not allow versions starting with non-digit when doing strict parsing, warn otherwise.
We have opened a bug against dpkg (#620566) as we believe that the new behavior in dpkg is more strict than the current Debian policy.
Also, we are currently working on updated Grml kernel 2.6.38 packages providing a version number that won't cause problems with dpkg 1.16.0 in the meanwhile.