i know its an old thread, but i think this is kinda connected.

i found that if you set the telephone-app to launch in portrait-mode, it WILL lock up the desktop. ALWAYS. at least thats fact for me, the last 4 times i tried it froze.
it doesnt happen if you set it to landscape or automatic.

also, killall hildon-desktop doesnt help too, because you have to restart the divice anyway.