They are, in server-side stuff--but not in desktop stuff, which is what Gnome 3 is specifically targeting. Ubuntu is far and away the largest desktop distro, so losing their support would be a pretty big blow to the adoption of the Gnome project as a whole--regardless of who's funding it.
This is becoming even more true as Steam will probably only support Ubuntu. Games are where the desktop users are. Gnome can spin its wheels and bask in its own self-righteous awesomeness but if the users aren't there, then it's all for nothing.
Canonical has the larger userbase, but their development work is pretty limited- basically, they do the Unity shell and their app-store. Most desktop related development (hardware support, DBus, core desktop libraries like PolicyKit, GTK+) happens inside Red Hat.
If they want to offer better binary compatibility between releases... well they can't, because they don't employ those guys.
If they want to support a new OpenGL release... well they can't, they don't employ those guys.
Ubuntu is crazy popular, but people really underestimate the extent to which they are an epiphenomenon.
Ubuntu conveniently distributes and packages desktop Linux. The software that they distribute may be written by a person employed by Red Hat, but I don't care. I just want to run a version of Linux that is easy to use, configure and upgrade.
>This is becoming even more true as Steam will probably only support Ubuntu.
Has Valve said that they will only be supporting Ubuntu, at the moment they are still in a private beta for the initial port to Linux, and if it goes well I suspect that we will see Valve support steam on non-Ubuntu systems. Even if they do not officially support other systems, I suspect we will see people make on-official ports.