According to Linux-KVM.com, Red Hat has recently started the development of a GTK client for Linux that leverages the open source SPICE remote desktop protocol.
The project still is in its very early stage, as version 0.2 for 64bit Fedora 14 is the only binary available at the moment.
SPICE too is far away from reaching version 1.0, expected not earlier than in H2 2011.