Hi Thomas,
The short answer is, we don't have a package for Debian. At the moment, the only platform we officially support is Ubuntu 12.04 LTS.
You could, however, try to install on Debian the package we have prepared; it should work, though I will let the other developers comment if that is not correct. Again, this is not officially supported, so do this at your own discretion. See:
https://github.com/gem/oq-engine/wiki/Ubuntu-12.04.
I hope that helps.
Lars