[sverrejoh]

You can also kill Safari (and all other running apps) by holding the sleep button until it shows the slider screen, and then hold the menu button until you get the home screen.

[Ivatar]

You can also bring up the task tray, press and hold the icon for safari(or any other app) like you would for moving apps on the home screen, then press the - sign that appears to close it. Discovered it by accident, but it's quite useful.

[thought_alarm]

Yes, but that should not be necessary unless an application is hung.

It's much easier and quicker to quit Safari by closing its tabs.