Apparently Quicksilver is your only third-party menu bar icon, and all
the others are built-ins.
Things like the clock, Airport icon, Volume control icon, Time Machine
icon, etc., are built into Mac OS X. Apple uses a special method for
those icons (which, among other things, allows them to be reordered by
Cmd-dragging). In the interest of preserving system stability, Apple
makes non-Apple software use a different method to put icons in the
menu bar. This means (1) that such icons cannot be mixed in with the
Apple ones, and (2) that Cmd-dragging does not work as a method to
reorder the icons within this group.
The groups of icons in the menu bar are always drawn in this order
(from right to left): Spotlight icon, other built-in icons, third
party icons. Within each section, it is possible to reorder the icons
by various means, but it is not possible to move icons from one
section to another, or to change the order of the sections.
So what Quicksilver's setting does is allow the Quicksilver icon to be
the rightmost icon in its section, but it will still always be to the
left of the built-in icons. And since Quicksilver is your only third
party icon, being first or last on a list of one makes no difference.
In short, only Apple could change this, and they aren't likely to.