Committed to the Plymouth master code-base and a branch of GDM is support for very nicely handing off control of the display(s) between Plymouth and GDM. With the Plymouth code found in Fedora 12 and earlier, it sort of assumes and hopes that the X Server starts up nicely from where Plymouth left off, but if that doesn't go smoothly, the user is left in a troubled state. Now though this has changed where Plymouth receives a deactivation signal, which rather than just killing Plymouth, it leaves the plug-in idling on the screen until the X.Org Server takes over and is properly initialized. The GDM then signals to Plymouth when to properly shut itself off and is switched over to showing the X Server.
The new work that hit the Plymouth tree can be found in this Git commit, which was previously part of Plymouth's no-fbcon branch. The Plymouth integration work for the GDM can be found in the plymouth-integration branch. Lastly, Red Hat's Ray Strode who has been the principal engineer of Plymouth has written a blog post covering this nice Plymouth-to-X hand-off process.