I'm not certain that this is strictly a matter for upstream developers but I
thought it worth reporting upstream just in case

I can only speak to my own experiences under Ubuntu 10.10 (x86-64) wherein the
visual transition between Plymouth and GDM is far from smooth. Typically this
visual transition entails an abrupt change to a black screen between plymouth
and GDM. I assume that this affects other distributions and display managers also, but since the Fedora developers approach (see below) required a patch to GDM I figured it was worth reporting against GDM.

If the following post to freedesktop's mailing lists
(http://lists.freedesktop.org/archives/plymouth/2009-June/000126.html) is to be
believed* this need not be the case. With a few patches to both Plymouth and
GDM they appear to have achieved a smooth transition of sorts. 

I'm not sure whether the Fedora developers approach is technically desirable,
but the current situation which entails a somewhat jarring and abrupt
transition to the display manager is sub-optimal in my opinion.

*I have no direct experience of Fedora