rhythmbox-0.12.4 has a udev useflag which should apparently send something about "gudev" to configure. But there is a typo in the ebuild. Will attach a patch
Created attachment 203416 [details, diff] rhythmbox_ebuild.patch
+08 Sep 2009; Gilles Dartiguelongue <eva@gentoo.org> + rhythmbox-0.12.4.ebuild: + Fix a typo in udev flag activation, bug #284037. It was fixed into the main tree by Gilles, without a bump. thanks for reporting