Quicksilver was purchased by Apple and put into OSX 10.3 several years ago.
Quicksilver was not purchased by Apple. In fact, the source code for QS is now open source (under an Apache license) and available at http://code.google.com/p/blacktree-alchemy/.
Apple's Spotlight is not a launcher per-se but more a system-wide search facility. It does work as a very simple application launcher (since apps are also indexed) - but definitely not a equivalent to QS or Gnome Do, which are essentially context sensitive mash-up of GUI based shells with a noun-verb model for operating on system objects such as applications (for launching, hiding, quitting etc.) and specific verb actions for tasks such as displaying contact info, opening chat sessions, operating on files etc.
A computer lets you make more mistakes faster than any other invention, with the possible exceptions of handguns and Tequilla. -- Mitch Ratcliffe