Same here.
As soon as I need to update or reinstall the Parallels Tools I lose Unity.
The last post on this thread resume my issue nicely:
http://forum.parallels.com/showthread.php?t=210307
I am on Ubuntu 11.10 and few kernel update late (to avoid to trigger the updating of Parallels Tools).
Edit: Macbook pro early 2011 with Parallels Build 7.0.15098 (Revision 770637; June 13, 2012)
Last edited: Jun 30, 2012