FWIW, based on the above comments and my experience I’d lay at least some of the blame on this on nouveau: I have two setups affected by the Plymouth no-show, both with Nvidia graphics:

01:00.0 VGA compatible controller: NVIDIA Corporation G73 [GeForce 7300 GT] (rev a2) (prog-if 00 [VGA controller])
and
00:0d.0 VGA compatible controller: NVIDIA Corporation C61 [GeForce 7025 / nForce 630a] (rev a2) (prog-if 00 [VGA controller])

At the same time all the other systems I have access to that run Precise on non-Nvidia graphics (some half dozen computers, old and new) don’t manifest it, i.e. they display Plymouth’s boot logo screen just fine on every boot without any workarounds. (The Nvidias need either FRAMEBUFFER=y or plymouth:force-drm.)

Also, on the G73, Grub menu shows at a low resolution without GRUB_GFXMODE specified, but on the C61 it’s ”Out of range”. It shows on both if I specify 1280×1024 (the maximum supported by the connected LCDs) as GRUB_GFXMODE. This too is Nvidia-specific, though I suppose the Grub menu is outside nouveau’s jurisdiction.