On my desk I have few ARM based computers: PandaBoards, Efika MX Smartbook, BeagleBoards. Each of them is running same operating system (Ubuntu), none of them has OS fine-tuned except of kernel and x11 drivers (which is a case on PC too).
Also have other ARM systems which I would not call embedded as I can run Debian on them.
But I also have (small) experience with embedded x86 systems which OS was done in fire-and-forget way which is common to embedded market regardless of CPU architecture.