Mac Lion Prevent restart after a new software install or uninstall
You can install updates without restarting by running sudo softwareupdate -i -a. It might cause some issues though. When I installed the 10.8.3 update without restarting, Safari started crashing on launch until I restarted the Mac.

If an installer or uninstaller forces you to restart, you can often just press command-Q to quit the installer.

I haven't heard of anything like a soft restart. You can quit all applications by running sudo killall loginwindow, but it doesn't have the same effect as restarting or logging out and back in.