We are currently finishing the transition from a stdio-based GNU C Library (glibc) to a libio-based one. This is the result of about five months of work we put into getting the system ready and, of course, the work that the glibc developers did to make glibc what it is.
This change will have various advantages, for example libio has been tested more extensively, as it is also used by most GNU/Linux systems for some time now. However, it also means a change in the Application Binary Interface (ABI) of glibc, thus you will need to reinstall an existing Debian GNU/Hurd system. Upgrading has not been tested at all, so better do not expect it to work. Also note that you will need to get some of the Debian packages from alpha.gnu.org. Please read the recent mailing list archives for details.
Important Note: As another temporary complication, the current installation tarball is available at a different place than usual.