Found an answer on Macworld.com
You can achieve the same thing in QuickSilver with and some basic AppleScript:
tell application "Safari" make new document tell application "Safari" to activate end tell
Simply save this AppleScript somewhere that QuickSilver will find it.
The good thing about the above AppleScript is that it doesn't mess around with focus (which can inadvertently switch Spaces). Unfortunately not all applications have a similar AppleScript command for making a new window (for example, Terminal does not).
I saved mine in my home directory, and called it "New Safari Window". I also changed it so that it had the Safari icon (by dragging the Safari icon from the "Get Info" window of Safari.app, to the icon in the "Get Info" window of the new script file).