I know this may not be the way you wish to handle the urpmi/rpmdrake freeze or lock, in fact you may have discovered already, but the only fast way to unlock it is to reboot. I assure you it will unlock during the reboot.
Now the inly way to prevent the lock from happening is to limit your installs to only 5 to 10 packages unless they very large or very small. In such cases you would use less or more. You still may have problems depending on how much memory is sucked up before you started and such, but my suggestion works.
Have fun anyway.