The trouble began when I suddenly lost the ability to use Mac triggers under Windows when in fullscreen mode in Parallels. So I couldn't do things like use Command+Tab to switch to other applications, use the Fn keys for expose and, my favorite, use the trigger for VirtueDesktops to switch from fullscreen Windows to OS X.
My desperate hunt for that elusive option I changed somewhere that messed things up ended when I discovered this post in the Parallel's forum. Apparently, enabling the Enable access for assistive devices option under Universal Access has this side effect. Unfortunately, QuickSilver needs this option enabled for its keyboard triggers feature to work. I was using that feature to bind a keyboard shortcut to maximize windows in OS X.
Well, in the end Parallels won out. I can live without the maximize shortcut but not without my Mac hot keys under Parallels. It would be nice if they could all just play nice and get along though! 
The Parallels vs. QuickSilver article by Aral Balkan, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial 2.0 UK: England License.
Hello,
Any idea if this has been fixed? I am now using QuickSilver a lot and considering installing Parallels to use Windows on my MBP (Intel).
Thanks,
Olivier
I had the same problem. I had been noticing that my parallels moves *really* slowly, so I had decided to open activity monitor. while parallels was open it showed me that quicksilver was hogging like 50-100% of cpu. when I quit quicksilver, everything worked nice and quickly.