For me, there are two stumbling blocks for a multiseat workstation in current Linux distros (e.g. Fedora 17): pulseaudio and gdm/systemd.
For pulseaudio, the situation is actually not so bad: PA has been originally designed as a flexible multi-purpose tool. It is possible to run PA in system mode, and use a single system-wide daemon to split a 6-channel on-board sound card to three independent dual-channel outputs. The problem is that system-wide mode is not supported (even discouraged in the PA documentation), and for example Fedora does not even contain the init-script (or .service file) for system-wide PA, and it also does not contain suitable SELinux policy module for it.
The other problem is this shiny new systemd/gdm "autoconfiguration". It would be nice for those rare cases, where everything works as authors intended. My problem is that the Xorg.conf file is used only for the primary seat, and for other seats, a temporary minimal file is generated under /run, with no control over its contents and no control over Xorg(1) command-line arguments. For example, my ~3 years old home workstation with two Radeon HD 3450 boards needs the -isolateDevice command-line argument, and there is no way to set it with systemd/gdm. I have been using manual setup and xdm for the last several years, which works without problems.
So from my point of view, systemd definitely does not enable the multiseat capability. This capability has been there for at least ten years, and works without (if not despite) systemd.
FWIW, there is 5+ years old Fedora Bugzilla entry about gdm not being able to pass the arbitrary Xorg command-line options:
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds