I may have answered you earlier, but I answered in another thread that I just checked on my Vista PC by changing the Hosts file and OneCare did not stop me or alert me to the changes. So, it appears that at least for the Host file change, initiated by me and providing administrator rights via the UAC prompt, OneCare allowed me to do it. I don't know if OneCare will prevent another program from changing it, but I suspect that it should - in that this is malicious behavior - which OneCare monitors for.
-steve