The second beneficiary is the open-source community. The Gnome interface has grown into a useful windowing system and the Linux community has taken to it. Sun distributing Gnome with Solaris (as opposed to shipping with its upcoming Linux release) amounts to a large commitment by a major vendor. That should increase the work on the Gnome project and put an increased quality and usability requirement on the interface.
Gnome is a high-quality desktop. I use it every day, and the problems I have are not with windowing. Sun has spent years building a reputation for quality and stability, and the vendor would not get involved with an unstable project.
Sun isn't the first company to integrate open-source software into its offerings, nor will it be the last. But Sun's weight adds credibility in a way that other vendor endorsements can't.