While Linux is most popularly considered a family of operating systems, Free Software Foundations purists reserve that name for the kernel, or the very core of the operating system. They prefer to call the resultant distribution of kernel and software running on the kernel as GNU/Linux since it is a combination of GNU software and the Linux kernel. Debian is thus known as Debian GNU/Linux.
GNU Hurd on the other hand is an alternative kernel that the GNU software was originally written for. GNU Hurd provides the services that implement file systems, network protocols etc. over the GNU Mack microkernel. It is still a work in progress despite being in development for over 20 years.
Debain is known to be widely ported, being available for as many as nine architectures: i386, amd64, powerpc, sparc, mips, mipsel, ia64 s390 and armel. In the previous Debian 6 release, the distribution added the option of using a FreeBSD kernel (i386 or amd64) with the Debain software collection, and in the next version they intend to provide similar support for the GNU Hurd kernel.
It is unlikely that Debain based on Hurd will be able to replace the Linux-based version in the short term considering that it still supports very few devices. Still, Debian 7 isn’t out for quite a long time (2012/2013), and things might still change. In the end open source is all about choice, and this just adds one more to the list.
You can already download a version of GNU/Hurd to play around with if you are so inclined.