He in fact points to a freedesktop url so that display manager writers can learn how to hook into the systemd specific bits correctly to get access to this.
He's not thumbing his nose at anyone. He's not gloating. He's going... this is cool... and we have it working in GNOME's GDM in Fedora 17... we'd love for other display managers and other distributions to get this working too. We've even provided some high level instructional material for developers of other display managers to help them figure out what they need to do to hook in.
Oh and if you need hardware to test...they can send up some demo units of the pluggable hardware to test. I mean holy crap... offers of free hardware to test against. He's bending over backwards to make it as easy as possible for other implementations to getting support for this coded in.
If the kdm developers want this feature...its not going to be a big deal for them to add. If the developers of lightdm wants this feature...they can probably get it as well. However, I don't expect lightdm developers to make it a priority, as its very Unity specific and if Ubuntu doesn't make the jump to systemd there will be very little pressure to get lightdm extended to support a systemd specific capability. But stranger things have happened.
If Canonical wants to really go after the enterprise desktop space, this would seem like an important feature. Being able to really position Ubuntu better in the thin client market... and maybe even the point of sale markets. This functionality is a business win. The fact that Canonical doesn't see that is troubling. Especially if this works out of the box with livecds...and you can demo the functionality from a cold boot using some random laptop at a meeting with a customer with no icky low level configuration. Enterprise oriented sales teams must be drooling over this.
And FYI, the key here is GDM... not strictly limited to "GNOME the desktop". I'm pretty sure xfce as a session under GDM in Fedora 17 works just fine. Unity2d would probably work fine as well..if there was a systemd using distribution shipping a Unity2d session definition for gdm to expose.