Some users are reporting an issue where invoking QS causes Spaces to switch to the space where QS's command window last appeared.
On my computer at least, the fix is simple. In the Spaces system preference pane, set Quicksilver to appear on every space. As an added benefit, mouse triggers will also work on every space when this setting is enabled. Note that on 10.5 with β54 this setting did not work properly with Quicksilver, but 10.6 and/or β56a7 has fixed whatever the problem was.
