The Hurd servers themselves are multithreaded, so they should be able to take benefit of the parallelism brought by SMP/Multicore boxes. This has however never been tested yet because of the following.
Mach used to be running on SMP boxes like the Intel iPSC/860, so principally has the required infrastructure. It has however not yet been enhanced to support nowadays' SMP standards like ACPI, etc. Also, GNU Mach's Linux device driver glue code likely isn't SMP-safe. As this glue code layer is not used in the Xen port of GNU Mach, the plan is to try it in this enviroment first.
That is why for now GNU/Hurd will only use one logical processor (i.e. one core or one thread, depending on the socket type).